aDB PDO like database abstraction

This class implements a database abstraction layer similar to PDO.

It comes with a base class that provides an interface to the basic database access wrapper functionality.

Drivers that provide the actual implementation of the database access functionality should implement a sub-class of the main interface class.

The main interface class provides functions for:

- Establishing connections
- Executing queries
- Set query result range limits
- Prepared queries
- Transaction management

Result sets can be retrieved with iterators.

Database access errors are handled with exceptions.