Add Ember.computed.limit and use it in LimitedArray

arrayComputed was added recently in order to make array computations
easier. Using arrayComputed we can improve LimitedArray which now uses
an isolated multipurpose Ember.computed.limit.
Use Em.computed.sort and Em.computed.limit instead of nesting proxies

With new array computed helpers we can simplify code and get rid of
nested array proxies.