v0.2

This is major release that includes some new features and quite a few bug
fixes. Here are the highlights:

• There is now a direct constructor for DataArray objects, which makes it
  possible to create a DataArray without using a Dataset. This is highlighted
  in the refreshed tutorial.
• You can perform aggregation operations like mean directly on
  xray.Dataset objects, thanks to Joe Hamman. These aggregation
  methods also worked on grouped datasets.
• xray now works on Python 2.6, thanks to Anna Kuznetsova.
• A number of methods and attributes were given more sensible (usually shorter)
  names: labeled -> sel, indexed -> isel, select ->
  select_vars, unselect -> drop_vars, dimensions -> dims,
  coordinates -> coords, attributes -> attrs.
• New Dataset.load_data and Dataset.close methods for datasets facilitate lower level of control of data loaded from disk.