Update README.md

The updates are:

* Indicate that it is possible to pass a block to `super` to add custom
  behaviour to some Devise controller actions without completely
  overriding them.

* Move mailer configuration out of the section "Controller filters and
  helpers".

* Consistently use Ruby 1.9's hash syntax, highlight code and capitalize
  "Devise".

* Wording improvements
Merge pull request #2866 from guilhermesimoes/readme-update

Readme update