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)
I'm wondering if there's a way we can make this more ruby-esque and either:

a) use a symbol like `:filter` that takes a hash, then convert it to the appropriate MYOB variable like `$filter` in the `url` method. e.g. `(:filter => {:name => company_file[:name]})`

or

b) use methods like `.filter` that take the filter properties like .filter({:name => company_file[:name]})

Not sure what is best, would love some feedback though!