major (php-)refactoring and documentation updates

I have been refactoring our backend, application structure and bootstrapping in the last few weeks.

In the backends, we extend the abstract sql backend class which contains all the standard functions like create, update, delete, etc. and is able to handle the different records and database tables.

Our application structure also changed a little bit. I updated the documentation in the wiki.

The bootstrapping process has been revamped as well. We splitted the Tinebase_Controller and created a new Tinebase_Core class which now does all the initialization work and can be used to get data from the registry (like the database adapter, current user, ...).

More new wiki articles cover the Exceptions concept and a HowTo for building a Tine 2.0 application (still work in progress).