Allow page parts to use customised presenters

A page part will look for a named presenter and named data.
If found it will use the presenter to present the data.

Collection_presenter is a new presenter which will present any
collection as an unordered list.

This has passed the refinerycms testing, but I haven't written tests for the new behaviour yet.
Also, I'm not sure if what you have here works without custom presenters for every project that wants to use this. This won't detect a CollectionPresenter because it's not a CollectionSectionPresenter.