Smarter recognition of Python files

[carlio]

(moving from landscapeio/landscape-issues#67)

Currently prospector looks for files with a .py extension, but this could be a bit smarter. For example, first use mimetype to look for text files, then inspect those for a python hashbang or other indications of python-ness.

[guykisel]

You might be able to use Pygment's guesser - http://pygments.org/docs/api/?highlight=guess#pygments.lexers.guess_lexer_for_filename