Expose bytearray

Instead of implementing the SysV MQ API with go strings, make it work with byteslices internally. This way binary data is supported. 

- Expose the bytelsice API using `ReceiveBytes` and `SendBytes`. 
- `Send` and `Receive` still work with go strings.



@graemej @benbjohnson @Sirupsen @snormore Please check if I am doing memory management properly.