ADD: Added test for ComputeTubeGraphProbability.

     Added test + MIDAS keys.
ENH: Use Midas3FunctionAddTest(NAME ... COMMAND ...) syntax

     Removed explicitly setting TextCompareCommand and using the
     variable in Midas3FunctionAddTest in testing the TubeTK app.
     ComputeTubeGraphProbability (as Matt suggested).
Merge pull request #105 from rkwitt/Test-ComputeTubeGraphProbability

ADD: Added test for ComputeTubeGraphProbability
BUG: Fix memory leak in ImageToTubeRigidMetric