Don't allow indexing like DataArray.coords[0]

Relevant for #197
Switch to noncoords from noncoordinates
Remove "to" argument from Dataset.apply
Require specifying attrs with a keyword argument in Dataset.__init__

Lays groundwork for #197
Fix documentation for coord related changes
Switch .as_index to .to_index()
Add DataArray.to_dataset()
Remove Index alias
Only allow tuples (no lists) for implicit parameter Variable conversion
Update xray internals documentation note
Another pass through the API docs
Update What's New
Merge pull request #213 from shoyer/checklist-for-v0.2.0

Checklist for v0.2.0