Use pkg_resources to get the version from setup.py

needed by the developer (base requirements + testing tools +
documentation tools)

Add documentation building to the tox and travis test scripts.

to test all of the code on all of the configurations, in much the same fashion
as Travis-CI does.

Add support for GroundOverlay

coercing to a valid type or defaulting to None.

Added GroundOverlay from string test case that uses a sample kml from Google reference documentation.

* Attempt to solve issues with Python 2.6 unittesting when using the:
    with self.assertRaises(...): ...
  By specifying "as context" on the with statement