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)
This line will break things for existing users. I'd prefer to have a separate API for this. Maybe:

````ruby
def records(params=nil)
    response = all(params)
    response.is_a?(Hash) && response.key?('Items') ? response['Items'] : response
end

Thoughts?