Consistent names: always use 'attrs' rather than 'attributes'

Related: #190
Rename coordinates -> coords and dimensions -> dims

The only breakage should be for anyone who has been using the
new DataArray constructor. Every other use of 'dimension' or 'coordinates'
should work but raise a FutureWarning.
Remove use of coordinates and dimensions in tests
Switch documentation to dims and coords
Merge pull request #194 from shoyer/consistent-names

Consistently use shorter names: always use 'attrs', 'coords' and 'dims'