Fix Db_Abstract E_STRICT errors (issue #41)

[bertrandmalet]

abstract static methods raise E_STRICT errors since php 5.2. Created an interface.

[gametaunt]

an interface with static functions still makes no sense

[staabm]

alle static methods of the DB Abstract method should be made non-static and therefore all the subclasses need to be adjusted accordingly.

[bertrandmalet]

I agree that all static methods shouldn't be static. But it was a first step to introduce the db layer. Like preinheimer said before must of the code is kinda messy and probably need a good cleanup, just time missing. Need to check was excalty change to remove the static methods.