Add tests, documentation, and implement the Valuer interface

This PR does way too much:
- Updates `all` make target to execute tests
- Removes old junk from .travis.yml
- Implements driver.Valuer interface for big.Int/Rat wrappers
- Adds tests for Scan and Value for the big.Int/Rat wrappers and NullTime
- Modifies big.Int/Rat wrapper's Scan to accept more types than just string
- Adds documentation