Database Schema Cleanup

This is a first cut at cleaning up the database schema some.  Overall changes:

- Remove unused `#__languages` and `#__session` tables
- Adding FKs as appropriate
- Reordering the table order in the installer to work with the FKs

The last unused table is the `#__categories` table which was being used for field data in the item submit/edit form in the Platform version of the app.  Left to decide is whether we still use this data or if we just drop it in favor of the pure Markdown interface in place now.

Image below shows the altered schema (as compared to the one I [posted to G+](https://plus.google.com/u/0/117560389441437562876/posts/Ea4JFRAQw9m) the other day)

![jtracker_cleanup](https://f.cloud.github.com/assets/368545/904821/dfb0f6bc-fbdb-11e2-8e01-ff055343c896.png)