Remove $NTA reference

Supports https://github.com/numenta/nupic/issues/809.

This PR should be merged together with https://github.com/numenta/nupic/pull/917 (pip-installable). This is because `lib_cppregion.so` will be installed too. In the current implementation, this file is obtained from $NTA/lib.
@scottpurdy I didn't understand well your doubt, but I'll answer from I understood. In the pip PR, NuPIC is installed into a system default location. However in this location there's no `lib` folder which I should put `libcpp_region` library into it, but only in the `nupic` folder itself..