Silence warnings + use Werror

this PR finalizes the silence warnings PR merged to nupic.core. The build process is very clean (only 1 numpy warning), let's us even run with Werror for better code quality demands. Adds some flexibility to our CMakeLists file (compiler specific flags - yes, somewhere are needed)

Tested on gcc 4.6, 4.7, 4.8 and clang.