Support __iter__ and __len__ for utils.ChainMap
Support coords in Dataset constructor
Coordinates no longer are restricted to indexes

All tests pass but more coverage and documentation is needed.
Allow indexing datasets with lists of variables

Fixes #193
Deprecate DataArray.dataset
Update DataArray.concat to take all Dataset.concat parameters
Add xray.concat; deprecated Dataset.concat and DataArray.concat
Support returning an array from DatasetGroupBy (and vice-versa)

Fixes #218
alignment cleanups
Add DataArray.reset_coords
Fix Dataset.__getitem__ for coordinates
Tweak conditions for showing values in Dataset.__repr__

Now, (a few) values are always shown for a netCDF file on disk.
Check edge cases for Dataset.from_dataframe
DataArray constructor edge cases
Deprecate select_vars and DataArray.drop_vars
Fix Python 2.6 compat
more tests and fixes for coordinates
Many more tests for coordinate math (and some fixes)
Documentation updates
Fix python 3.4 regression