Reverse on :db/isComponent attributes only returns a single entity.

Refer:
 - https://groups.google.com/forum/#!searchin/datomic/reverse$20get/datomic/Eu7l__rA-xk/MWqTTECbJI0J
Merge pull request #138 from scott-abernethy/develop

Reverse on :db/isComponent attributes has a special case for reading.