Improve the page navigation block

**Note:** This PR requires #47  to be merged first.

The page navigation block only allows an absolute page tree or a relative tree below the current page. This PR adds the ability to have relative tree that includes the siblings of the current page.

This PR is branched off the Django 1.7 support and therefore add both South and Django migrations. The South migrations add a datamigration for backwards compatibility, the Django migrations don't because nobody is supposed to use them yet (makes the migrations cleaner).