ENH: Dataset.reindex_like and DatasetArray.reindex_like

This provides an interface for re-indexing a dataset or dataset array using
the coordinates from another object. Missing values along any coordinate are
replaced by `NaN`.

This method is directly based on the pandas method `DataFrame.reindex_like`
(and the related series and panel variants). Eventually, I would like to build
upon this functionality to add a `join` method to `xray.align` with the
possible values `{'outer', 'inner', 'left', 'right'}`, just like
`DataFrame.align`.

This PR depends on PR #71, since I use its improved `copy` method for
datasets.