use Sort vararg for ordering of query results

[cstrempfer]

Instead of
datastore.find(Rectangle.class).order("width,-height").get();
you can now write
datastore.find(Rectangle.class).order(ascending("width"), descending("height")).get();

This partially solves #735. I didn't implement methods to chain sort parameters.

The Sort class is in org.mongodb.morphia.aggregation right now. It could be moved to org.mongodb.morphia.utils as it's used for queries and aggregation now, but that will break existing code.

[evanchooly]

Thanks! This looks great. I have a few tweaks I want to make to address the package issue you mentioned but I think we can get this merged next week.

[cstrempfer]

@evanchooly I've made the changes you suggested in #499. It wasn't necessary to change the type of Sort#order (aka. Sort#direction).

I didn't change the unit tests of sorting in an aggregation pipeline to make sure the old API still works.