Change default of ArrayGroupBy.reduce to dimension=None

This makes xray consistent with pandas: `obj.groupby('year').sum()`
should return an object with 'year' as a dimension, not an object
where the 'year' dimension is summed out.

As a bonus, the implementation is simpler (less code).
Merge pull request #108 from akleeman/groupby-reduce-default

Change default of ArrayGroupBy.reduce to dimension=None