Refactored CMakeLists.txt to better support a workflow that treats nupic..core as an external library.

re: https://github.com/numenta/nupic.core/issues/77

This is an API-breaking change.  `CMAKE_INSTALL_PREFIX` is used to specify the installation location rather than defaulting to `build/release`.

    mkdir -p build/scripts
    cd build/scripts
    cmake ../../src -DCMAKE_INSTALL_PREFIX=../release
    make install

The result will be a fully functional nupic.core installation, including statically compiled `libnupic_core.a` in `build/release/lib` and headers in `build/release/include`, which may be re-used externally (as part of a nupic build, for example).

This change is not compatible with nupic as is.  A separate complementary PR will be submitted to nupic as well.