Add 'restrict' template keyword to tabulate_tensor signatures.

Setting 'restrict':'' will be as before, 'restrict':'__restrict__'
will inject '__restrict__' in all the pointer arguments.
This will allow testing of the compiler specific keyword for performance.
improve CMake's Python finding

fixes bug https://bugs.launchpad.net/ufc/+bug/1155621
don't require CMake 2.8.9
don't explicitly set PythonInterp_FIND_VERSION
Merged in nschloe/ufc (pull request #1)

improve CMake's Python finding