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
After that, I think it should be good to go, my only concern is if someone intentionally named something with a leading number and underscore, but that feels too edge case to worry about right now.  And in that case, we'd only be stripping off the number underscore that we prepended to theirs, so it should be fine.

Merge away when you're ready.