Add support for cracking GNOME Keyring files

I have added support for cracking GNOME Keyring files to hashkill.

Test File: Extract sample Keyring file from http://openwall.info/wiki/_media/john/openwall.keyring.tar archive.

```
✗ hashkill --plugin keyring -f openwall.keyring -r dictionary -a ~/wordlist
[hashkill] Version 0.3.1
[hashkill] Plugin 'keyring' loaded successfully
...

[hashkill] -= Cracked list =-

Username: 		Hash: 					Preimage:
-----------------------------------------------------------------------------------
openwall.keyr 	GNOME Keyring file                       	openwall             

[hashkill] Total: 1 passwords cracked
...
```

It would be great if someone could port this plug-in to OpenCL.