Create and show for dataset collections

Hey John,

I've been working a bit on dataset collections and I managed to get `HistoriesClient.showDatasetCollection()` and `HistoriesClient.createDatasetCollection()` implemented.  I based the structure of the code on how collections are implemented in bioblend http://bioblend.readthedocs.org/en/latest/api_docs/galaxy/all.html#module-bioblend.galaxy.histories.  There's some examples in https://github.com/apetkau/blend4j/blob/feature/collections/src/test/java/com/github/jmchilton/blend4j/galaxy/Examples.java#L246 and integration tests in https://github.com/apetkau/blend4j/blob/feature/collections/src/test/java/com/github/jmchilton/blend4j/galaxy/HistoriesTest.java.

So far this is working for me, but I was interested in getting some feedback on whether or not this is how you were thinking of structuring the Dataset Collections API in blend4j.

Thanks,

Aaron