Improve CSS helper

[fphilipe]

• Code improvements
• No longer load default CSS file when <style type="text/css"> tag is present

Test by adding this to your Gemfile:

gem 'premailer-rails3', git: 'git://github.com/fphilipe/premailer-rails3.git', branch: 'improve-css-helper'

[thbar]

Hey - just tried that on our brand new staging server:

• removed email.css
• used your branch

I can confirm the email is delivered with the expected look in gmail via sendgrid.

Thanks for crafting this!