Make the python module installation directory configurable.

Advanced cache variable VTK_INSTALL_PYTHON_MODULE_DIR now controls
where the install target puts the python .py files and corresponding
python extension libraries (*Python.so or *Python.dll). ThirdParty
modules like Zope, Twisted and Autobahn should be placed there,
vtk extensions should go inside the vtk directory there.

System installers for instance will likely want to set it to be:
/usr/local/lib/pythonX.Y/site-packages #Linux
/Library/Python/X.Y/site-packages #Mac
C:/PythonXY/Lib/site-packages #Windows

ParaView will use it to put the VTK python modules underneath its own.
Also for ParaView's use, there is once again a way to disable installation
of the vtkpython binary. Now it is done by setting VTK_INSTALL_NO_PYTHON_EXES.

The change also updates the README.txt per VTK 6.X layout and
removes references to setup.py, which we no longer support.

Change-Id: I92c1d65c8ca8aa44ca2d54cc4526bbc72f42eb0d
Merge topic 'improve-vtk-python-installation' into master

bec28326 Make the python module installation directory configurable.