Make this repo pip-installable

[tbekolay]

This requires organizing the files under a good root with an __init__.py, and writing a setup.py.

A few things that need to be hashed out before that can be done:

• How do we deal with extra dependencies some examples might have?
• How do we deal with different filetypes? .ipynb I'm looking at you.

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Some thought to kick off the discussion...

For the first point, we should definitely have nengo and nengo_gui as requirements in setup.py, but probably only those two. For examples that use other packages (nengo_spinnaker, etc), we should have some helper functions in this repo to check if certain things are installed and give helpful messages if they're not.

For the second point, I think for the time being we just make sure to include them in MANIFEST.in so that they show up when the repo is pip installed. In the future, it would be nice to include some helper functions or something that can run .ipynb files or that can load up the GUI with the given .py file etc.