README: link to gh-pages Doxygen Documentation
New link
Updated cmake MPI variables

 - use new MPI_C/CXX_ variables
Merge pull request #20 from ax3l/topic/onlineDoxy

README: link to gh-pages Doxygen Documentation
Update INSTALL.md

fix INSTALL_PREFIX flag
General documentation update.
Merge branch 'master' of github.com:ComputationalRadiationPhysics/libSplash
Remove abs path from title