only push denormalize_to changes if the field is dirty (improved)

- Improved https://github.com/logandk/mongoid_denormalize/pull/12 based on code review comments.
- Assorted other code style improvements.
- Added information about :as option to the README, plus info about getting started with jeweler.

If you don't have time to accept my PR, please consider giving me access to accept PRs! If you like the quality of this PR, maybe you will trust me to give your project a little breath of life. http://felixge.de/2013/03/11/the-pull-request-hack.html