Add support for checksum verification for the otultra2 driver.

This introduced some changes in the Exception class to simplify the
code, and at the same time it adds a module for multiple lifescan
drivers to share code (multiple OneTouch protocols share the same
checksum for instance).
Implement the correct checksum algorithm.

Unlike what I though, OneTouch2 does not use CRC32 Sick algorithm, but
a very stupid full 16-bit modulo sum of the bytes in the reply.

This is very simple and does not require GPLv3 code at all.
Relicense everything under MIT license.

There is no reason for me to use GPLv3 now that I'm not using the
CRC32-Sick code, so I'll go with the most liberal license out there.