Add entity class and interface

I can remove tests from the PR if they make it too big, but I think they may help show the entity methods in action.

The real focus of scrutiny on this PR really should be in the page class and the migration for the table setup.

1. Outstanding issue to figure out: How to enforce unique "route" names.
Ah, that's right, preg_match uses the strange &$matches...

If $route != '' is for the regular epression, it should be run before preg_match in the same if rather than after, also noting why that is there in a comment would be a good idea.