fix protobuf decode repeated string

when libpomelo decode "repeated string", it will decode as empty objects like [{},{},...].
the original code is
![bd78e1d4-e9c1-49a4-9e47-4ec73dcf7712](https://cloud.githubusercontent.com/assets/2226090/5261681/3a42b3cc-7a57-11e4-9dc7-dbabd054dacb.png)

seems like you intended to decode string as an object, but that do not conform to son format. so I wonder the why doing this ?

another possible fix is to modify pomelo-protobuf/lib/encoder.js, treat string as a simpleType and encoding its length, original code attached:
![0fb14364-9a66-4491-b0ff-d145756a39a9](https://cloud.githubusercontent.com/assets/2226090/5261749/2fb5d78a-7a58-11e4-9a05-d7e772fbcb2e.png)