add Lita::Handlers::Github.whois command

This adds a whois command for the main Github handler for looking up information on a GitHub user:

`!gh whois theckman` or `!gh user theckman`

They return:

```
theckman (Tim Heckman) :: https://github.com/theckman
Located: San Francisco, CA
Company: PagerDuty, Inc.
Orgs: PagerDuty
ID: 787332
GitHub Admin: false, Repos: 40, Gists: 1
Following: 13, Followers: 18, Created: 2011-05-14 04:16:33 UTC
```

This is helpful when adding a new user to a team. Will allow you to confirm, via a public channel, that you found the right user.
Merge pull request #24 from PagerDuty/whois_user

add Lita::Handlers::Github.whois command