Implement filtration on backend side

* add possibility to filter, sort and limit data(see [here](http://developer.myob.com/api/accountright/api-overview/retrieving-data))
* add convenient `find` method to find by `UID`
* cleanup code(use new hash syntax for consistency, remove `{}` when hash passed as last arg)
I'd really prefer that myob-api use the rocket syntax for keys, thanks!