Add I2C support and refactor platform detection to common module.

Bumping the library up from v 0.1.0 to v 0.2.0.  The only public interface change to code from the previous version is the get_platform_gpio function no longer takes a plat parameter with the string platform name.  This parameter was only used for injecting specific platform names in during testing, and this test code has been refactored to use the Mock library to inject a platform value.