ENH: Use virtualenv for Base/ and Applications/ Python tests.

The tubetk Python module is installed into the Python testing virtualenv with
a .pth file and the virtualenv is used for the Base/ and Applications/ Python
tests.  No modifications to the local system are required anymore to test the tubetk
python package.
Merge pull request #87 from thewtex/BasePythonTests

ENH: Use virtualenv for Base/ and Applications/ Python tests.
ADD: Test for TubesToImage

     Added test for TubeTK application TubesToImage.