Complete Crawler. Close #15, #14, #11, #8.

Several of the closed issues were addressed partly in previous commits;
definitively close them with this, for the moment, final update to the crawler
package.

Ref:
    bitshift/crawler/indexer.py
            -move all `GitIndexer` specific functions (eg, `_decode`,
            `_is_ascii()`)from the global scope to the class definition.