Proposed fix for issue #5

Replace gem manifest with bundler

See http://addons.heroku.com/sendgrid

Gem manifest has been deprecated in favour of Bundler, see
http://devcenter.heroku.com/articles/gems

Readme Documentation cleanup

* Started with fixing "Your all Set" to the correct "You're all Set"
* added markdown tweaks to make the requirements a list
* add emphasis for *reviewthis*
* changes to "Roll Your Own" section to break out Heroku vs non-Heroku

Basic HTML email template included. Also now parsing every commit passed in the payload, rather than just the newest. #reviewthis @jwelshiv @tonydewan

If a commit message includes the hash #reviewthis, any github user
(signified by @username) will get an email (if they have a public
email address). Also, any email address includes gets an email as
well.

Currently, sendmail is used in dev, and since we're going to deploy
to heroku, the sendgrid addon is used in production.