TODO: release v0.8.0

[shoyer]

I've been putting this off long enough -- we need to get all the recent work on xarray out there!

Things I'd like to merge first:

• xarray.merge function and major refactor for merge logic xarray.merge function and major refactor for merge logic #857
• Fix issue with expand_dims on 0d arrays of tuples Fix issue with expand_dims on 0d arrays of tuples #867
• Refer users to Stack Overflow for support in the docs Doc fixes #922

Given the scope of these internal changes (especially #857), possibly a release candidate would be in order?