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.
@DavidRagazzi This is temporary, right? If so, what is the plan to replace it with something that doesn't depend on python? 

Sorry, I think we discussed this a week ago, but I can't remember what the future plan is.