A single CMake file with all build information

A new and improved CMake file.

Improvements:

Replacement of all CMake files to a single file in order to ease understanding and maintenance (this avoids several files scattered in the repository).
Environment variables (PATH, PYTHONPATH, NTA_ROOTDIR, etc) are set in own CMake file.
Now it use the own Swig executable and dependencies located on External folder. Before this, I used 'find_packages' which could set an incompatible version for Nupic.
Now it use the system python, i.e. the python package called from shell natively (similar to what Autotool version does). Before this, I used 'find_packages' which could set wrong python interpreter or libraries.
Build result: SUCCESS (including Swig modules!)

Tests results:
HtmTest: SUCCESS in all tests!
TestEverything: SUCCESS in all tests!
Python tests: SUCCESS in all tests!
Merge pull request #514 from DavidRagazzi/patch-4

A single CMake file with all build information