Optionally specify pre-built nupic.core installation without rebuilding

NuPIC analogue to https://github.com/numenta/nupic.core/pull/170

To use (and re-use) nupic.core installation:

1. Clone and build nupic.core:

        git clone https://github.com/numenta/nupic.core
        mkdir -p nupic.core/build/scripts
        cd nupic.core/build/scripts
        cmake ../../src
        make

2. Build NuPIC, specifying nupic.core location:

        python setup.py develop --cmake_options="-DNUPIC_CORE=..."

    Where the value of `NUPIC_CORE` is the path to the installation directory of nupic.core (presumably `<path to nupic.core repo>/build/release` assuming `-DCMAKE_INSTALL_PREFIX` was not specified as described in numenta/nupic.core#170

Under this scheme, nupic.core will not be cloned and built locally as part of the nupic build.  If `-DNUPIC_CORE` is not specified, then nupic.core will be cloned and built normally.  This is an alternative to the .nupic_modules/.nupic_config scheme which is left in-tact.

cc: @scottpurdy re: numenta/nupic#1234