Major revamp of the documentation

Preview the built docs: http://xray.readthedocs.org/en/docs-v2/

Feedback would be greatly appreciated!
Would it be crazy to merge the concepts of "dimensions" and "coordinates"?  I'm used to thinking of coordinates as having a name (e.g., "right ascension"), units (e.g., "arcseconds"), and providing the tick labels.  The coordinates that index the ndarray would be simple "coordinates", and additional coordinates could, borrowing from Iris, be "auxiliary coordinates".