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.
I find this inherently risky. Depending on the configuration of the site the user can edit the names of sections. If they accidentally changed the name or thought "Employees List" was a better name than "Collection" it would break their page with no visibility as to why.

Page templates are a similar functionality for pages and when they are turned on they are accessed by a clearly marked dropdown in the advanced section. It is quite clear what has changed if you change that and the page breaks. Of course, sections don't have an advanced section, but possibly a better option would be to extend the UI instead of hacking into what is available.