Making linking against Python simpler on Windows.

This patch deprecates the three Python CMake variables in favor of
a single variable PYTHON_HOME which points to the root of a python
installation. Since building Python doesn't output the files in
a structure that is compatible with the PYTHONHOME environment
variable, we also provide a script install_custom_python.py which
will copy the output of a custom python build to the correct
directory structure.

The supported workflow after this patch will be to build python
once for each configuration and architecture {Debug,Release} x {x86,x64}
and then run the script. Then run CMake specifying -DPYTHON_HOME=<path>

The first time you do this will probably require you to delete your
CMake cache.

The old workflow is still supported during a transitionary period,
but a warning is printed at CMake time, and this will eventually
be removed.

Differential Revision: http://reviews.llvm.org/D8979