Fixed bug with handling multiple ws messages packed into one tcp frame.

Here is complete fix to following situations: 
* Buffer contains parsed part of message and control should go back to loop to read more data. (#21)
* Two (or more?) ws messages were packed into one tcp frame, so we need to parse (retrieve_frame) buffer again after message was handled.
* Readed data contains final part of one message and first part of another message, then after handling first message remain is set back to undefined (https://github.com/jeremyong/websocket_client/blob/master/src/websocket_client.erl#L344 and https://github.com/jeremyong/websocket_client/blob/master/src/websocket_client.erl#L362), so part of next message will be parsed correctly before going back to loop.