Build docs on RTD using conda

[shoyer]

It works!

To preview, see http://xray.readthedocs.org/en/rtd-conda/

Fixes #602

[jhamman]

Looks good. Stoked to see the current docs again.

1. Can we add cartopy to the docs build. This would allow us to build the docs with map examples.
2. Do we have to build the docs with Python 2.7? If not, I'd suggest we standardize on Python 3.

[shoyer]

@jhamman I think those are both good ideas. Do you want to try them in a follow-up PR?

[jhamman]

Sure, that's fine. I think you can merge this.