reCaptcha support on forms

Hi Matt,

This is a rather big patch, so have a look and let me know if there is anything you would prefer to do another way.
I did some basic testing and it seems to be working correctly.
This patch covers four parts:
1.- UI for form parameters: there is another tab in configuration for form information (in the future we could add more because reCaptcha has some interesting parameters)
2.- UI editor: for adding the widget to the form and moving it around
3.- UI web: generate the code in the form for the site
4.- Backend form validation

I can setup a demo or explain anything you may have doubts about.

I did find one error that I left for you because I had a look but couldn't find it and you have fixed it in the past so I suppose it will be easy for you: if you leave the widget menu open in the editor and press "Save", the icons appear in the form on the website. You have to make sure all widget menu are closed.