Add class ChangeDir, amend unsafe subprocess.
Add class ChangeDir, amend unsafe subprocess.

Add:
    bitshift/crawler/git_indexer.py
        -add ChangeDir class, a context-management wrapper for os.chdir().
        -replace unsafe "rm -rf" subprocess call with shutil.rmtree()

earwig/bitshift,earwig/bitshift,earwig/bitshift,earwig/bitshift,earwig/bitshift,earwig/bitshift