Contact Forms: Prefix all fields with a number.

Prefixing all fields by a running number let's us keep the current simple storing format while supporting having multiple fields of the same name and type. These prefixes are removed in the admin view, as well as the success message (though only if the prefix exists to not break previous feedback in admin).

Addresses https://github.com/Automattic/jetpack/issues/145
Alrighty. Thanks for the feedback!

Yeah I wouldn't worry about that breaking anything, because we only strip out the underscores when displaying the actual field labels. Stripping the prefix off of any past manually prefixed labels isn't something I'd be concerned about either.