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
```
I'm afraid I'm not sure what you're referring to. Is there a std library function that will parse out the key in this format? `ssh.ParsePublicKey` is unable to parse a key like

`ssh-rsa AAAAB3NzaC1yc2E...`

The key part `AAAAB3NzaC1yc2E...` must be separated out and base64 decoded first.

Is there a different function I should be using? Regardless, I agree that I can go about this without using regexp.