Remove some subprocesses, comment out logging.

Add:
    bitshift/crawler/
        (crawler, indexer).py
            -comment out all logging statements, as they may be causing a
            memory leak (the crawler is meant to run perpetually, meaning that,
            depending on how the `logging` module is implemented, it may be
            accumulating logged strings in memory.)

        bitshift/crawler/indexer.py
            -make `_index_repository()` and `_index_repository_codelets()`
            functions of the `GitIndexer` class.
            -replace `_get_tracked_files()` subprocess call, which found the
            files in a Git repository and removed any that were non-ASCII, with
            a pure Python solution.
            -add `_is_ascii()`.