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.
is there an open feature request with bitbucket? theyre usually pretty good about adding stuff to the API.