BUG: Fix datetime components on DataArrays

We didn't have any tests, so they were broken.
Merge pull request #228 from shoyer/fix-time-components

BUG: Fix datetime components on DataArrays
test for dataarray.groupby.reduce with keep_attrs=True
Refactor ops.py to handle binary only ops (groupby)
Support binary arithmetic operations with groupby objects
Refactor for utils.array_equiv
Minor optimization for Coordinates._merge_validate
ensure virtual variables remain coordinates
Merge pull request #229 from shoyer/groupby-math

Support math with GroupBy objects
change dataset iteration to includes vars and coords

Per my offline discussions with @akleeman, to iterate only
over variables, one should now use `Dataset.vars`.
Rename Dataset._variables -> Dataset._arrays

Related: #212

Dataset.variables was too confusing with our current usage of
"variables", but I'm not 100% sure I'm happy with `Dataset.arrays`
either.

So for now, I've just renamed `Dataset._variables` to
`Dataset._arrays` and removed `Dataset.variables` entirely.
Merge pull request #232 from shoyer/vars

Revert dataset iteration to include vars and coords
WIP: docs for v0.3