Page changes

This pull request adds two new features to the Page class:

Ajax Templates/get_template method
You can now set an ajax template by setting the ajax_template attribute on a page model. Wagtails default serve method will use this template instead of the standard one for ajax requests. If you want more advanced behaviour, you can override the new get_template method.

get_context method
Instead of overriding serve, you can now just override get_context and return only the context variables you need. This is useful when you have a subclass of a custom page type and want to reuse that page types context with some modifications. 

Note: This pull request previously contained another feature which allows urlconfs to be inserted into pages. This will be added into a separate module.

The original pull request can now be found on my page-changes-2 branch.