Remove git submodule in favor of CMake control

Removed nta git submodule, replaced with CMake commands to manually clone, fetch, and checkout nupic.core repository.  This also allows overriding the defaults on the command line via NUPIC_CORE_REMOTE, for the remote repository url, and NUPIC_CORE_COMMITISH, for the specific sha.

There are several advantages to this approach:

1. No git submodules (nupic git repository is no longer explicitly coupled to git://github.com/numenta/nupic.core.git)
2. CMakeLists.txt is the ground truth for nupic.core version dependency
3. User can override NUPIC_CORE_REMOTE and NUPIC_CORE_COMMITISH at-will, if they are not using the official nupic.core repository.
4. Developer can make changes in nta/ and push to remote repository of their choosing