Removing nta dir with all C++.
Added nupic.core as local submodule at "nta".

This replaces the previously removed `nta` directory with the nupic.core
codebase and names it `nta` to replace the deleted code.
Updated build script to refresh core submodule...

before building.
Added cleanup of nupic.core submodule after build.

The build leaves nupic.core submodule in an altered state because of the
files it dumps. This resets all submodules to initial state after
building.
Using secure URL for submodule
Cleanup build script re: submodule.

Trying to plan for different states. If a user removes the nta dir
submodule, this will put it back before initializing the submodule. Also
cleans up after build in a cleanup function.
Disabling Travis's submodule support.
Trying another URL for submodule.

Per
http://docs.travis-ci.com/user/build-configuration/#Use-Public-URLs-For-Submodules.
Fixing submodule url
Merge pull request #597 from rhyolight/core-submodule

Core submodule