Expose more information in DataArray.__repr__

This PR changes the `DataArray` representation so that it displays more of the
information associated with a data array:

- "Coordinates" are indicated by their name and the `repr` of the
  corresponding pandas.Index object (to indicate how they are used as
  indices).
- "Linked" dataset variables are also listed.
   * These are other variables in the dataset associated with a DataArray
     which are also indexed along with the DataArray.
   * They accessible from the `dataset` attribute or by indexing the data
     array with a string.
   * Perhaps their most convenient aspect is that they enable [`groupby`
     operations by
     name](http://xray.readthedocs.org/en/latest/tutorial.html#apply) for
     DataArray objets.
   * This is an admitedly somewhat confusing (though convenient) notion that I
     am considering [removing](https://github.com/xray-
     pydata/xray/issues/117), but we if we don't remove them we should
     certainly expose their existence more clearly, given the potential
     benefits in expressiveness and costs in performance.

Questions to resolve:

- Is "Linked dataset variables" the best name for these?
- Perhaps it would be useful to show more information about these linked
  variables, such as their dimensions and/or shape?

Examples of the new repr are on nbviewer:
http://nbviewer.ipython.org/gist/shoyer/94936e5b71613683d95a