Refactored CMakeLists.txt to use nupic.core as an external library

NuPIC complement to numenta/nupic.core#228

This is an API-breaking change to nupic with a refactored CMakeLists.txt to need only access to a nupic.core installation prefix which is specified via -DNUPIC_CORE cmake option (as before), but no longer requires access to the raw source code repository.  Howeverm if -DNUPIC_CORE_SOURCE is specified, that overrides the value in .nupic_modules for repository location should nupic.core need to be build inside the nupic source tree.

*Note*: I'd like to be able to re-write the history in this one to exclude mention of my own fork in .nupic_modules.  Once ddc622bf99c886692f2b19892a56ecc70e8fa5de is resolvable from git://github.com/numenta/nupic.core.git I will make that change and redo this PR.