db layer rewrite

add bridge class for easier future replacement of db layer

new interface throws DbException when errors occour and logging is performed by db layer, not needed to do it in application level.

also this branch tries to use more placeholders where possible (some code was too difficult to change without  breaking it easily)

this is WIP branch, will be submitting more commits here
[ ] add new bridge code which keeps current PEAR::DB api
[ ] use exceptions
[ ] replace code to use exceptions
[ ] drop APP_DEFAULT_DB and APP_TABLE_PREFIX from core code
[ ] replace confusing calls getCol and getAssoc with alternatives