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

Fixes #221, #222, and #225 

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).  To create a binary tarball release, call `make package`, which will yield a .tar.gz with the version name and platform in the filename.

This change is not compatible with nupic as is.  See https://github.com/numenta/nupic/pull/1519 for complementary nupic PR.