users can set/update the label, description, license, and attribution per dataset

users can set/update the label, description, license, and attribution per dataset, http://bamboo.io/docs/advanced_commands.html#updating-dataset-metadata
 * new endpoint PUT datasets/[ID]/info

Implemented as `dataset.set_info(attribution=None, description=None, license=None, label=None)`

Only non-None parameters are updated. Empty strings allowed.