Add GitHub repo star count, simple logging.

Add:
    bitshift/crawler/crawler.py
        -add `_get_repo_stars()` to `GitHubCrawler`, which queries the GitHub
        API for the number of a stars that a given repository has.
        -log the `next_api_url` every time it's generated by `GitHubCrawler` and
        `BitbucketCrawler` to two respective log-files.