[WIP] Javascript Validation

[beberlei]

Making a pull request out of this so discussion is simpler.

Idea:

1. Have a javascript library that mimcs the validation rules of the server side Symfony validation.
2. Have a way to expose certain parts of the validation metadata to the client side.
3. Combine validators and metadata to allow client side validation.

[stof]

you should not add trailing whitespaces :)