Refactoring of LastPass post module

There were two primary goals with this refactoring:

* Clean up the code a bit
* Store the browser from which the LastPass creds were looted

Tested it on Linux w/ Chrome and Firefox:

```
msf post(lastpass_creds) > run

[*] Searching for LastPass databases...
[*] Found user: jhart
[*] Checking in Chrome...
[+] Found 1 database/s in Chrome
[*] Checking in Firefox...
[+] Found 1 profile files in Firefox
[*] Looking for credentials in all databases found...
[*] Decrypting password for user jon_hart@rapid7.com from Chrome...
[*] Decrypting password for user jon_hart@rapid7.com from Firefox...
[+] LastPass credentials
====================

 Username             Password                Browser
 --------             --------                -------
 jon_hart@rapid7.com  blah1    Chrome
 jon_hart@rapid7.com  blah2  Firefox
```

On Windows with Chrome, Firefox, Safari and Opera:

```
msf post(lastpass_creds) > run

[*] Searching for LastPass databases...
[*] Found user: user
[*] Found user: cyg_server
[*] Found user: Administrator
[*] Checking in Chrome...
[+] Found 1 database/s in Chrome
[*] Checking in Firefox...
[+] Found 1 profile files in Firefox
[*] Checking in Opera...
[+] Found 1 database/s in Opera
[*] Checking in Safari...
[+] Found 1 database/s in Safari
[*] Looking for credentials in all databases found...
[*] Decrypting password for user jon_hart@rapid7.com from Chrome...
[*] Decrypting password for user jon_hart@rapid7.com from Opera...
[*] Decrypting password for user jon_hart@rapid7.com from Safari...
[+] LastPass credentials
====================

 Username             Password              Browser
 --------             --------              -------
 jon_hart@rapid7.com  blah  Chrome
 jon_hart@rapid7.com  blah  Opera
 jon_hart@rapid7.com  blah  Safari
```
(No, blah is not my password -- I changed it)

Extraction from Firefox 33 on Windows isn't, but it doesn't appear to have worked in the original PR either (it is also hard to tell because the creds in the original didn't retain the browser from which they were obtained).  The issue seems to be a different format, perhaps between Windows/Linux or different versions.  

With Firefox 33 on Linux, my prefs file looks like:

```
user_pref("extensions.lastpass.loginpws", "jon_hart%40rapid7.com=<blob>");
user_pref("extensions.lastpass.loginusers", "jon_hart%40rapid7.com");
```

Whereas Firefox 33 on Windows:

```
user_pref("extensions.lastpass.loginpws", "<base64something>");
user_pref("extensions.lastpass.loginusers", "jon_hart%40rapid7.com");
```

I haven't had time to dig into this further, but land this PR in your repo and then rapid7/metasploit-framework/#4004 will be nearly landable.