Data array constructor

Fixes #115.

Related: #116, #117.

Note: a remaining major task will be to rewrite/reorganize the docs to introduce `DataArray` first, entirely independently of `Dataset`. This will make it easier for new users to figure out how to get started with xray, since DataArray is much simpler.
On the other hand, `ds.indexes` is currently just a plain `OrderedDict`. It would be nice to have consistency here... but I'm not quite sure what the right API for both is.