Add GitHub stars, Bitbucket watchers; close #14.

Add:
    bitshift/crawler/crawler.py
        -Add more efficient method of querying GitHub's API for stargazer
        counts, by batching 25 repositories per request.
        -Add watcher counts for Bitbucket repositories, by querying the
        Bitbucket API once per repository (inefficient, but the API in question
        isn't sufficiently robust to accommodate a better approach, and Git
        repositories surface so infrequently that there shouldn't be any query
        limit problems).