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
Switch setup.py version back to 0.2.0-dev
addition of Dataset.reduce methods
Merge pull request #137 from jhamman/dataset_reductions

Dataset.reduce methods
Add keep_attrs to reduction methods
simplify copying of attributes and keep_attrs tests
clean up test style and add a bit of documentation
update signatures of numpy reduction functions
Merge pull request #141 from jhamman/keep_attrs

Add keep_attrs to reduction methods
Merge pull request #128 from xray/data-array-repr

Expose more information in DataArray.__repr__
Build docs with IPython==2.1.0

This lets use lines starting with `Out[XX]:` in verbatim output thanks to the
fix ipython/ipython#5816.
Use "equivalence" for all dictionary equality checks

This should fix a bug @mgarvert encountered with concatenating variables with
different array attributes.

In the process of fixing this issue, I encountered and fixed another bug with
utils.remove_incompatible_items.
Merge pull request #144 from shoyer/equivalent

Use "equivalence" for all dictionary equality checks
Remove install_requires if on readthedocs

This is the simplest solution I can think of for my documentation build woes.
More stupid fixes for readthedocs
Merge pull request #145 from xray/fix-rtfd

Fix doc builds on ReadTheDocs
Support "None" as a variable name and use it as a default

This makes the xray API a little more similar to pandas, which
makes heavy use of `name = None` for objects that can but don't always
have names like Series and Index.

It will be a particular useful option to have around when we add
a direct constructor for DataArray objects (#115). For now, arrays will
probably only end up being named `None` if they are the result of some
mathematical operation where the name could be ambiguous.
Fix Python 3 failures comparing None to strings
Merge pull request #147 from shoyer/variable-None

Support "None" as a variable name and use it as a default
Constructor to create DataArrays from scratch