When using Mysql2 client (MRI), don't pass timezone to DateTime columns

thanks @kjeremy! fixes #24
Update Rakefile so you can specify a single adapter on the command line for tests to be run against.
Merge pull request #27 from pnomolos/testing_tweaks

Update Rakefile so you can specify a single adapter on the command line ...
Merge branch 'master' into null_equality
Bump version and add notes to change log