.loc and .sel support indexing with boolean arrays

Fixes #182
One more test for boolean .loc
Raise NotImplementedError when attempting to use a pandas.MultiIndex

Related: #164
Merge pull request #195 from shoyer/loc-booleans

.loc and .sel support indexing with boolean arrays
Merge pull request #196 from shoyer/raise-on-multiindex

Raise NotImplementedError when attempting to use a pandas.MultiIndex
Cleanup of DataArray constructor / Dataset.__getitem__

Now Dataset.__getitem__ raises a KeyError when it can't find a variable.
Merge pull request #198 from shoyer/dataarray-constructor-fix

Cleanup of DataArray constructor / Dataset.__getitem__
Fix renaming in-place bug with virtual variables

This is why mutating state is a bad idea.
Merge pull request #201 from shoyer/bug-fixes

Fix renaming in-place bug with virtual variables
Major revamp of the documentation
Tweaked project overview docs
Raise an error when attempting to use a scalar variable as a dimension

If 'x' was a scalar variable in a dataset and you set a new variable with
'x' as a dimension, you could end up with a broken Dataset object.
Merge pull request #207 from shoyer/raise-on-incompat-scalar

Raise an error when attempting to use a scalar variable as a dimension
Fix Dataset.argmin

Fixes #205.

Includes a failing test (currently skipped) for #209.
Merge pull request #210 from shoyer/fix-dataset-reduce

Fix Dataset.argmin
Doc edits per Todd's feedback
Add note to encourage getting in touch
Merge pull request #202 from xray/docs-v2

Major revamp of the documentation
Consistent names: always use 'attrs' rather than 'attributes'

Related: #190
Rename coordinates -> coords and dimensions -> dims

The only breakage should be for anyone who has been using the
new DataArray constructor. Every other use of 'dimension' or 'coordinates'
should work but raise a FutureWarning.