Initial BitBucket implementation.

Implemented the BitBucket issues api. There are two methods of authentication available:
- username and password (the same as for Jira issue tracker)
- Oauth 1.0a 2-legged authentication. I added two parameters to the argument list:
  - the BitBucket consumer key
  - the BitBucket secret consumer key.

OAuthBase class is from [googles' Oauth](https://code.google.com/p/oauth/) library. 

I added RestSharp package. Tell me if that is ok. I can try to implement it with HttpWebRequests if you do not wish to add the dependeny.