Added base class for Overlay
Added implementation of icon setter w/ tests.
Added test coverage for Overlay class
* Added members for Ground Overlay tags.
* Implemented the altitude property and etree_element() functionality.
Finished implementation of the GroundOverlay class. All that is still needed is the <gx:latLonQuad> tag.
Added test coverage to Feature > address and phoneNumber tags
Added TODO for <gx:latLonQuad>
* Added "from_string" functionality to the _Overlay and GroundOverlay classes.
* Attempt to solve issues with Python 2.6 unittesting when using the:
    with self.assertRaises(...): ...
  By specifying "as context" on the with statement
Second attempt to make unittest work with Python 2.6 by using unittest2 backport
Only install unittest2 if Python 2.6
Update changelog to include new GroundOverlay support.
Added GroundOverlay from string test case that uses a sample kml from Google reference documentation.
Raise ValueError on setting bad Overlay / GroundOverlay members instead of
coercing to a valid type or defaulting to None.
Merge pull request #17 from IanLee1521/master

Add support for GroundOverlay