Update Textpattern_Server_Var.

* Rename Var, since its a keyword (ns f/c), to Config.
* Remove magic get (to avoid syntax collisions).
* …and introduce getVariable() method.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5728 2fea6a4d-a838-0410-917b-93a53c48e9d2