Mov author_files > git_inder, heavily refactor.

Add:
    bitshift/crawler/crawler.py
        -add base crawler module
        -add github(), to index Github.
Mod:
    bitshift/crawler/
        -add package subdirectory for the crawler module, and any subsidiary
        modules (eg, git_indexer).

    bitshift/author_files.py > bitshift/crawler/git_indexer.py
        -rename the module to "git_indexer", to better reflect its use.
        -convert from stand-alone script to a module whose functions integrate
        cleanly with the rest of the application.
        -add all necessary, tested functions, with Sphinx documentation.