Remove TransactionOptions wrapper

Since you've gone down the path of eliminating your wrappers for datastore.Key, Query and the other structs, I think you should also stop wrapping TransactionOptions. That allows users to just use datastore.TransactionOptions and be able to switch between nds and regular datastore operations.
Changed nds.TransactionOptions to datastore.TransactionOptions
Merge pull request #22 from derekperkins/patch-1

Remove TransactionOptions wrapper.