Teach CMake to create usable python package for the python STP interface
and to run tests against this package.
Teach CMake to install python package for STP inferface.

By default we try to guess where to install by asking
${PYTHON_EXECUTABLE} where its python library directory is (this for the
case that a virtualenv python is being use). This might might be a
little counter-intuitive because ${CMAKE_INSTALL_PREFIX} is completly
ignored during this process so a user without root access may end up
with a failing install by default.  A user can override what is detected
by setting the newly added PYTHON_LIB_INSTALL_DIR CMake option.
Disable python tests by default because the tests hit STP assertion
failures. This needs to be fixed properly at some point.
Merge pull request #105 from delcypher/python_cmake

Allow new python interface to be tested and installed. The tests are disabled though due to hitting STP assertion failures.