Fixes #11 (POST with list)

This *PR* fixes #11 for Exercise. I din't extend it for the other *kinds* as it is not well implemented. How to proceed will depend on a answer [here](http://stackoverflow.com/questions/21815401/post-list-of-objects-w-endpoints-proto-datastore) and also some research over the endpoints-proto-datastore code for generating the ProtoRPC messages and the inverse.