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(always use old style hash syntax for consistency, remove `{}` when hash passed as last arg)
Just be aware of the new pagination methods that were recently added too, see https://github.com/davidlumley/myob-api/commit/e80295e22d01bf65bab7278e4f165c99542b69c6 and https://github.com/davidlumley/myob-api/commit/ccdbc4585e29c7ad594f56c166a24e5545d6776d