support chaining within trigger groups, refs #22
fix a couple little issues

1. don't display warnings when generating schema.rb, since it's just a
   lot of noise with no context
2. don't choke on 1.9 syntax in schema.rb
3. allow create_trigger to be called w/ a name and no options
clarify behavior of name + trigger groups, fixes #22

also issue warnings when name is (apparently) used incorrectly for the
current adapter