Add mailcheck to email inputs

[callumacrae]

https://github.com/mailcheck/mailcheck

Apparently it's lowered sign up email bounce rates for the business that developed it by 50%: http://blog.kicksend.com/how-we-decreas ... ces-by-50/

Plus, it's awesome.

[PayBas]

If we simply change all the email inputs for prosilver to <input type="email" /> this could be a nice extension.

edit: seems we already did that