Add tested indexer.

Add:
    bitshift/crawler/indexer.py
        -add _debug().
        -add content to the module docstring; add documentation to GitIndexer,
        and the functions that were lacking it.
        -add another perl one-liner to supplement the `git clone` subprocess
        call, which terminates it after a set amount of time (should it have
        frozen) -- fixes a major bug that caused the entire indexer to hang.