Several improvements to the page module.
* [Page] Blocks now have priorities.
* [Page] When adding blocks, they are no longer separated by section (though they are still ordered by section).
* [Page] Bundles can now contain other bundles.
* [Page] Bundles are now loaded before page blocks.
* [Page] The default request now loads the "core" bundle. This means that when a module wants to load the rest of the page, it will only load those in the "core" bundle, while the Page module will load all blocks in the "default" module (which will also have the "core" bundle included during installation).