Added new extend for page types which allows assigning fields to specific pages

Ok I think this is ready to go.

[Here is a video demo of the PR](http://cl.ly/0L0g2i2R1W3b)

This adds:

* New "Page Type" option under Extend
* New dropdown in custom field for page type
* New dropdown in pages add/edit to assign a page type

You could also use this to create more verbose layouts considering you don't have to check for slugs/ids/titles etc, you can just check the page type and serve a partial or show/hide a section.