Can now whitelist users by github pubkeys

Addresses issue #46. Users can now whitelist others based on pubkeys from github as follows:

```
[USER] /whitelist github.com/$GITHUB_USER
-> Added github.com/$GITHUB_USER to the whitelist
[USER] /whitelist github.com/$BAD_GITHUB_USER
-> Error adding to whitelist: No github user github.com/$BAD_GITHUB_USER
```
ssh.ParsePublicKey will accept key formats described [here](http://tools.ietf.org/html/rfc4253#page-14), which includes `ssh-dss`, it that what you mean?

Ps. Regex has been removed as of a1455a8