Bump everything to recent Scala and Lift.

For now we’re still aiming for Lift 2.6 compatibility.
Add withFields and withAjaxFields macros.

These add an arbitrary list of fields to a base
FormalityFormProto and then immediately formalize or
ajaxFormalize it (repsectively), so that onSuccess can be
called on it directly.

This makes the Formality.form interface significantly cleaner.
README updated accordingly, as is the Signup class in the
example project.