Pip installable

With this PR NuPIC can be a PIP installable package.

Other improvements with this PR:
- $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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Fixes #809 
Fixes #602
Fixes #603
Fixes #604

Replaces https://github.com/numenta/nupic/pull/880
Does this mean `NUPIC` is _required_ to be set in the environment at runtime?