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.
Should `list(array.indexes)` provide a list of the dimensions or index objects? I am inclined toward the later, which is closer to `pandas.Series` rather than `collections.OrderedDict`. In that case this would need to be changed.