Add #debug? method for testing.

This checks the same environment variable CHAINDRIVE_DEBUG.
Set Ruby 2.0.0 as project requirement.

In reality I do not plan on straying from 1.9.3 specification which is
what the minimum version is (now) for the gem. But for development and
testing I am going to stick with 2.0.0.
Change to the Apache 2.0 license.
Update the test and specs to run properly.