Make sure FormExtracotrs are disabled unless we extract from a form type

[Nyholm]

This will fix #71

Ping @noofaq could you confirm?

[noofaq]

I have checked your changes in my project and it seems to work better - I don't need to annotate "@ignore" 'label' keys in controller - thank you for that.

Small suggestions:

1. maybe better is to check if SF MajorVersion == 2 to use "display value" by default and any other to use "display keys" as I don't expect Symfony team to switch back in the imaginable future
2. I see that code for ensuring Form visitors are really in *Type context is duplicated in few visitor classes; as PHP>5.5 is targeted I would suggest to use it in a trait or to have abstract BaseTypeVisitor to reduce code duplication
3. composer.json contains branch alias "1.2-dev" what creates conflicts when used with php-translation/symfony-bundle so I had to workaround that in my own composer.json using alias "dev-master as 1.3-dev"

Nonetheless, thank you for quick fixes 👍

[Nyholm]

Awesome. Thank you for the review. Ive done the changes you suggested.