Merge remote-tracking branch 'rapns/master'
Improve compatibility with official active record API.

- change_column_null isn't officially supported (espacially in the jdbc h2 implementation)
- see http://api.rubyonrails.org/classes/ActiveRecord/Migration.html
Add support for testing the jdbch2 adapter.
Merge pull request #109 from equinux/h2_support

H2 support