Db

- Relatively major re-write to use database URLs by default and split
out drivers into seperate packages.

- Rename DriverMissing exception to more descriptive NativeDriverMissing.
- Rename MissingDriver driver to more descriptive MissingNativeDriver.
- Fix MissingNativeDriver's connect method.
- Document all exceptions in README.txt.
- Add CHANGES.txt file.

- Add NoDefaultDatabase and NoSuchDatabase to replace generic KeyErrors.