Fix #81 getting a datastore.entity by key or path

New can get a datastore.entity by key or path
#81 
`entity_pb` is necessary for `test_get_entity_odd_nonetype`. There will raise `NameError` if don't construct `entity_pb`
![screen shot 2014-12-02 at 11 14 27](https://cloud.githubusercontent.com/assets/5171607/5257489/95e4a14c-7a14-11e4-879f-78ac93d62e5c.png)