Dataset.apply method

[shoyer]

Dataset reduce methods (#131) suggested to me that it would be nice to support applying functions which map over all data arrays in a dataset. The signature of Dataset.apply could be modeled after GroupBy.apply and the implementation would be similar to #137 (but simpler).

For example, I should be able to write ds.apply(np.mean).

Note: It's still worth having #137 as a separate implementation because it can do some additional validation for dimensions and skip variables where the aggregation doesn't make sense.