Add invoke tasks
'invoke clean' cleans up doc/_build/
Connect c library, add unit tests

Renamed to _lib.c to keep track of things.  Don't see a good reason
(yet) to move the ctype wrapping to its own file, especially for one
function.

Added a utility function for getting a .so file's path, since
http://www.python.org/dev/peps/pep-3149/ changed it up.

version is in two places since I couldn't figure out how to load it
from __init__ before setup built _lib.so (which __init__ tried
to load).  I'll try to address this soon.
Bump version since kmeans function now works