Add partially integrated BitbucketCrawler().

Add:
    bitshift/crawler/
        __init__.py
            -Initialize 'BitbucketCrawler()' singleton.
            -Instantiate all thread instances on-the-fly in a 'threads' array, as
            opposed to individual named variables.

        crawler.py
            -Add 'BitbucketCrawler()', to crawl Bitbucket for repositories.
            -Not entirely tested for proper functionality.
            -The Bitbucket framework is not yet accounted for in
            'indexer._generate_file_url()'.