Hi guys!

Related to https://github.com/symfony/symfony/blob/master/UPGRADE-2.5.md#validator and https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md#validator - specifically, I'm talking about the new Validator API and options.

Needed changes:

• A) Update the ExecutionContextInterface namespaces (see Update Callback.rst #4161 and 2.5 Validation API changes #4233)
• B) Update the addViolationAt to buildViolation
• C) Add a cookbook entry to explain all of this stuff, and show how a bundle can support both versions
• D) Update the framework reference section with the framework.validation.api option.
• E) Replace usages of deprecated method validateValue() where needed.
• F) Change signature of using ->validate with a group argument: