[#3044] Furthering @WouterJ's work on the ExpressionLanguage component

Hi guys!

This furthers the work done in #3044 for the ExpressionLanguage component. Comments warmly welcomed! We also need to work rapidly on the framework parts of #3022.

I borrowed some text from @fabpot's blog post (http://symfony.com/blog/new-in-symfony-2-4-the-expressionlanguage-component). I don't think he'll mind, but credit to him on many things in this entry.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8913)
| Applies to    | 2.4/master
| Fixed tickets | #3022

Thanks!