Document xray internals / advanced API

[shoyer]

It would be useful to document the internal Variable class and the internal structure of Dataset and DataArray. This would be helpful for both new contributors and expert users, who might find Variable helpful as an advanced API.

I had some notes in an earlier version of the docs that could be adapted. Note, however, that the internal structure of DataArray changed in #648:
http://xray.readthedocs.org/en/v0.2/tutorial.html#notes-on-xray-s-internals

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[shoyer]

This is done in http://xarray.pydata.org/en/stable/internals.html