Multi-OS support in travis

I modified extensively the travis file to allow multi-os support
and to use more recent UBUNTU:
- I upgraded most of the tests to UBUNTU trusty (with gcc 4.8)
- For a couple of tests I switch to UBUNTU precise (with gcc 4.6)
  to be sure we keep compatibility with older compilers
- I removed clang tests from UBUNTU and added OSX tests (both serial and MPI)

Since UBUNTU trusty comes with doxygen 1.8.6, I install doxygen 1.8.7
from source. Notice that this was solved in UBUNTU precise by using
an external ppa database. I prefer install from source here
since in this way we can easily upgrade doxygen to use latest features.

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