Provide module user a way to override parser/maker

Hi there! Sorry for show up abruptly. I found this module very useful for me but in my usage the message format is different, so I forked your module and do the following modification:

- Change the registry of recepie from .factory to .provider, make the
$websocket service configurable.
- $websocketProvider provides two methods to override default
  maker/parser:
  - `set_message_maker(maker)`
  - `set_message_parser(parser)`

This modification is pretty transparent and backward-compatible, module user won't find any difference, so I wonder if this modification also make sense to you, if so, feels free to merge it (then I can switch to your version in my `package.json`)

Again, thanks for the great module.