Test crawler/indexer, complete documentation.

Add, Fix:
    bitshift/crawler/
        __init__.py
            -add module and crawl() docstrings.
            -add repository_queue size limit.

        git_indexer.py
            -add exception-handlers to error-prone code blocks (eg, those that
            decode raw strings)
            -add time limit on the `git clone` subprocess, which caused problems
            when, for instance, cloning a corrupted repository, because it'd
            never exit. The current time-limit solution (another Perl one-liner)
            is un-Pythonic, but nothing else seemed to be working well with the
            indexer's threads.
            -add _decode()

        crawler.py
            -account for time spent executing an API query in the run() loop
            sleep() interval.