First part of #8 fix.

Add:
    bitshift/crawler/indexer.py
        -Add 'pkill git' to the 'git clone' subprocess in '_clone_repository()',
        to kill hanging remotes -- it's un-Pythonic, but, thus far, the only
        method that's proved successful. The RAM problem still persists; the
        latest dry-run lasted 01:11:00 before terminating due to a lack of
        allocatable memory.
        -Add exception names to `logging` messages.

    bitshift/assets
        -Update 'tag()' docstring to current 'bitshift' standards (add a ':type'
        and ':rtype:' field).