Added BinaryData type with a custom JSON marshal/unmarshal

- marshals/unmarshals according to the spec:
  https://github.com/tavendo/WAMP/blob/master/spec/basic.md#binary-conversion-of-json-strings
- issue #49 

This doesn't do any magic, so users will have to use this type in their structs if they want to be spec-compliant. We can perhaps add in a magic function to do this dynamically.