Adding new content types by checking for what is registered

[ryanwelcher]

Based on feedback for my pull request #31. I have taken a different approach to adding in support for more post types for the Zoninator.

First, I have added 'page' to the default list. Then I am using get_post_types() with a filterable set of options to pull all public, non-built in post types. I have also added a filter to allow access to the list of post types just prior to adding them that will allow the user to remove post types and adding a post_type_exists() check in the loop.