Fixed language loading bugs and removed debug code.

* User language was not loaded directly after logging in.
* Default language was not loaded directly after logging out.
* Removed stupid debug code. Was there since 2011-Oct-5. Unnoticed.
  *sigh*
models.php now uses PDO (UNTESTED!)
Fixed PDO DSN.
setup now uses PDO
New dependencies
Merge branch 'replace-mysql-with-pdo' into develop
areyousure.html is gone
Fixed spelling mistage in translation...
Fixed referencing and constants.

* Creating a varaible reference at call time is not possbile any more.
* When defining a constant, we pass the name as a string now.
Fixed wrong variable names
Merge branch 'develop'
Changed version number.

I guess this is okay, since the move to PDO is something pretty huge.

A note for plugin authors:

Unless you accessed the database manually, nothing has changed for you.
The API and the database model is still the same.