scan an individual module

[faassen]

I have a need for a method scan_module() that scans just a single module and does not recurse down packages. So it is related to issue #7 from 2011, I think.

I've factored an invoke() and a scan_module() method out of scan(). I think this also makes the code more readable as a whole.

The tests pass on Python 2.6 and Python 2.7, but the test runner (before my changes) has issues with both PyPy and Python 3.3, see issues #35 and #36. Once those get fixed I can send a pull request.

[digitalresistor]

@faassen If you'd like to provide changes from your fork back, I'd be willing to review and get them out.