Add in support for OAuth authentication

The old Campfire API only supported basic HTTP authentication, the new one supports OAuth. If you have obtained an OAuth token from the main 37Signals API then you can use that same token to access any campfire room that the token has access to. 

Basic HTTP Authentication details: https://github.com/37signals/campfire-api#authentication

OAuth: https://github.com/37signals/api/blob/master/sections/authentication.md#oauth-2-from-scratch

This adds in the option to specify :oauth_token when creating a connection which then sets a header of "Authorization: Bearer #{oauth_token}" instead of using basic HTTP authentication.