Feature/history download dataset

Hello John,

I've added functionality to be able to download a dataset from Galaxy into your local filesystem.  This is in the `HistoriesClient.downloadDataset()` method, which is similar in functionality to the bioblend [download_dataset](http://bioblend.readthedocs.org/en/latest/api_docs/galaxy/all.html#bioblend.galaxy.histories.HistoryClient.download_dataset).

Let me know what you think,

Aaron