Add tools to be able to find version and release

I added a version.hpp and updated the version.py so that
one can find the version of the library either from the
c++ or the python (if there is python support).

  modified:   CMakeLists.txt
  modified:   cmake/TRIQSConfig.cmake.in
  modified:   pytriqs/version.py.in
  modified:   triqs/CMakeLists.txt
  new file:   triqs/version.hpp.in