Cmake. Fix #97 and #89

- C++14 mode not automatic. Was based on compiler version,
  but the version of the C++ lib also matters....
  Now, there is an explicit USE_CPP14 option [default =OFF]
  to set to compile in c++1y mode.
  Solve also #89, i.e. the flags is now in TRIQS_CXX_DEFINITIONS and
  therefore pass to applications (including ipython magic).

- Python_use_mpi4py is now ON by default.
  Rational : it has been used for a while by e.g. Thomas,
  and it is necessary on OS X (boost.mpi.python raises a lot of issues).
  Hence we put it as default.

- Rename the option Build_Triqs_General_Tools_Test to a simpler
  Build_Tests