Add untested threaded indexer/crawler prototype.

Additions are not tested and not yet documented.

Add:
    crawler.py
        -add threaded GitHubCrawler class, which interacts with a GitIndexer
        via a Queue.

    git_indexer.py
        -add threaded GitIndexer class, which interacts with GitHubCrawler via
        a Queue.
        -rename context-manager ChangeDir class to _ChangeDir, because it's
        essentially "private".

    __init__.py
        -add body to crawl(), which creates instances of GitHubCrawler and
        GitIndexer and starts them.