Travis CI integration

- this required some modifications to the CMakeLists.txt file
  to support Release and Debug builds with different flags
  for clang and gcc
- travis-CI works only on Linux VM, that is, libcxx is not included
  by default in the llvm toolchain on linux and has to be installed
  from source when the compiler is clang.
- travis-CI linux virtual machines offer only up to gcc-4.8