0.3.0 work

- lib/how_slow/collector.rb
  - Cleanup of indentation/style
- lib/how_slow/setup.rb
  - Collapsed nested hash of various options down to a single-level
  - Removed the ::configure method as all Railties come with this by default
- lib/how_slow/mailer.rb
  - Changed the #metrics_email method to use the new collapsed setup options