Pip installable

With this PR NuPIC can be a PIP installable package.

Other improvements with this PR:
- Default directories structure for python projects
- NuPIC python files also can be edited directly from IDE! I love coloured syntax :heart_eyes: 
- $NTA was replaced fully by $NUPIC
- $PYTHONPATH was discarded as an user can use the `install` or  `develop` options from `setuptools`..
- $NTAX_DEVELOPER_BUILD also was discarded due the same reason above.

Please note, that $NTA can discarded as now everything can be done in a single location (`distutils/setuptools` experts here know what I mean).. This is possible due to distutils allow we install the NuPIC library in different places (PYTHON_PATH, HOME, etc)..

To have a better idea, I suggest read first the instructions from README..

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Solves #809

Replace this: https://github.com/numenta/nupic/pull/880