Add Remmina RDP/SSH/VNC password gathering

This is a more recent version of #3599, which went south after a bad merge and trying to get things reworked to  use the new credentials model.

The other day I had to use Remmina, a utility on Linux that (among other things) supports Network Layer Authentication (NLA) for RDP.  The system in question was configured to only allow RDP with NLA so I had no choice but to use this client.  I was surprised to find that it saved my credentials and didn't use a keyring/etc. As it turns out, this is on purpose -- http://remmina.sourceforge.net/faq.shtml.  So I tossed together a module to rip out my creds, because the though of having to use a client that puts my credentials at risk just to connect to a *more* secure host just feels wrong. 

# Validation

 - [ ] Install Remmina (available for (at least) Ubuntu 12.04, OS X)
 - [ ] Start Remmina, add a few saved connections of different types
 - [ ] Run this module after getting a session.  In non-verbose mode:


Locally, this looks like:


```
sf post(remmina_creds) > run

Remmina Credentials
===================

 Host        Port  Service  User      Password
 ----        ----  -------  ----      --------
 rdp2        3389  rdp      rdpuser2  rdppass2
 rdp3        3389  rdp      rdpuser3  rdppass3
 rdphost1    3389  rdp      rdpuser1  rdppass1
 vncserver1  5900  vnc      vncuser1  vncpass1

[*] Post module execution completed
msf post(remmina_creds) > set VERBOSE true
VERBOSE => true
msf post(remmina_creds) > run

[*] Searching for Remmina creds in 38 user directories
[*] Extracted secret Y8jO4HRbVjdSL7XugAHiI0nujS7PjJf4jUj1qtueFD4= from /home/jhart/.remmina/remmina.pref
[-] No password in /home/jhart/.remmina/1406846543539.remmina
[-] No password in /home/jhart/.remmina/1406846324986.remmina
[+] Collected 4 sets of Remmina credentials
Remmina Credentials
===================

 Host        Port  Service  User      Password
 ----        ----  -------  ----      --------
 rdp2        3389  rdp      rdpuser2  rdppass2
 rdp3        3389  rdp      rdpuser3  rdppass3
 rdphost1    3389  rdp      rdpuser1  rdppass1
 vncserver1  5900  vnc      vncuser1  vncpass1

[*] Post module execution completed
```
Directory can't be nil when using meterpreter, otherwise:

```
msf post(remmina_creds) > run

[*] run dir...
[-] Post failed: Rex::Post::Meterpreter::RequestError stdapi_fs_ls: Operation failed: 2
[-] Call stack:
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/lib/rex/post/meterpreter/extensions/stdapi/fs/dir.rb:61:in `entries'
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/lib/msf/core/post/file.rb:39:in `dir'
```