Updated setup.py to be pure-Python on Windows

On Gittle issue #12, 'pip install gittle' fails on Windows unless Visual
Studio is installed. This change disables all dependencies that use C
extensions during installation on Windows.
Original args on 1st try. Pure Python args on 2nd.
Converted setup's kwargs to a plain dictionary.
Merge pull request #19 from jacebrowning/issue12

Updated setup.py to be pure-Python on Windows