* Replaced the mess of every database class implementing filedExists() is its own...

* Replaced the mess of every database class implementing filedExists() is its own way with one simple function in base class. Verified to work on MySQL, Postgres and SQLite.
* Fixed fieldInfo() on Postgres not using tableName() and thus failing for table user, for example.
* Made fieldInfo() on MySQL return false instead of throwing a query error if table does not exist. This is consistent with other databases' behaviour.