Removed old Python detection cruft.

None of this should be relevant as of CMake 2.8.7.
Bumped minimal Python version to 2.6.

Rationale: we already depend on it (e.g. some scripts use print_function).
Refactored NumPy detection.

Most importantly, added support for multiple include directores,
but also did some general cleanup.
Moved Sphinx detection outside of the PYTHONINTERP_FOUND block.

Who knows, maybe the user's Sphinx installation uses a Python we couldn't find.
Merge pull request #1340 from SpecLad:python-updates