[cmake] Add USE_CPP14 is TRIQS_Config

- for usage in applications
[gfs] Change bz tag to using the actual brillouin_zone class

- simpler to remember ...
- in this case, gf is specialized on the actual name of the domain.
[gfs] Various fixes, for paper
Correction
fix C++11 issue in clef
API change in mpi: rename allreduce -> all_reduce
[doc] Install on Ubuntu 14.04
[mpi] Work on draft

- add invoke2, works now for block_gf
- TODO: more systematic tests
- TODO: remove the mpi.hpp, include as higher heder like in gf
Fix previous commit for c++11
[mpi] Clean, rm mpi.hpp,  Fix #133, Fix #140

- no need to include mpi, it is automatic from the array, gf containers.
- Fix example from issue 133.
- remove old example code.
Merge branch '1.2-dev' of github.com:TRIQS/triqs into 1.2-dev