Use #remove_possible_method instead

Move sandbox to AR and pass the sandbox as a value. This way, other ORMs like Datamapper can also have their own sandboxing in rails console.

Fixed tests for new_plugin_generator.

If passed "foo", a gem "foo-rails" is added to the Gemfile
and "foo" and "foo_ujs" are required in the application
JavaScript manifest.

Added the mysql schema test to mysql2 adapter