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