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
```
Why this check? Won't decoding or parsePublicKey catch any invalid keys?