Changed RIOT/Makefile.include, Makefile.base, Makefile.cflags, and native/Makefile.include to compile C and C++ files.
added riot_and_cpp example
added sudo apt-get install g++-multilib to travis.yml
Merge pull request #981 from vanhieubk/nhat_mixCompiler

Another way to mix c and cpp in user app with RIOT [native]