ENH: Simplify ImageToTubeRigidMetric.

Change-Id: I865b5cc8e0a4904905963de285af7b644f48d1bf
ENH: ImageToTubeRigidMetric: use CompensateSummation.

Improves robustness.

Change-Id: I7100e7328223d34cd4525e875ce6aeea2fb393a6
BUG: Create Python virtualenv at configure time. #95.

The virtualenv needs to available to install the tubetk packages for testing.
Merge pull request #96 from thewtex/Virtualenv-at-configure

BUG: Create Python virtualenv at configure time. #95.
BUG: Explicit link against PYTHON_LIBRARIES for Mac.
Merge pull request #98 from thewtex/MacPythonModuleLibrary

BUG: Explicit link against PYTHON_LIBRARIES for Mac.
ENH: Factor out tube point weight calculation.

Change-Id: Ieea9acbd082daa5d1fd3c97a5b8593297ad43ed5
COMP: Check for svn executable when using superbuild
COMP: Use https instead of http for GitHub
STYLE: Minor CMake-related style fixes
ENH: Bump LIBSVM tag to match current HEAD
Merge pull request #100 from jamiesnape/CMakeSuperbuildFixes

COMP: Various minor superbuild-related fixes
FIX: Filename mismatch in TubeGraphToImage test.
Merge pull request #101 from rkwitt/Fix-Tests

FIX: Filename mismatch in TubeGraphToImage test.
Merge pull request #99 from thewtex/ModelPointWeights

Model point weights