add BitBucket remote under lib/remotes, add BitBucket to index.js so it ...

...is available as a remote, add bitbucket test. Note: BitBucket API does not have support for git trees via REST api

Notes:
- No tree functionality available for BitBucket API. What are the implications of this
- Not sure how to test redirect functionality

Looking for feedback about what else I can do to improve this/get this in. I am willing to support this code long term and handle related issues as more people start to use it.

I would also be curious if there is a way you can create an injection mechanism that would allow for index.js to not have to be modified, because I would like to make an effort to write this for CodePlex and maybe even an individually configurable one for Gitlab in the near future.