Spring cleaning some two-way binding code

There were some epic hacks in place to get two-way binding to work with checkbox name bindings (i.e. this sort of thing - `<input type='checkbox' name='{{colours}}' value='red'>`), because multiple checkbox inputs that share a `name` attribute in effect share a single binding. Since #696, those hacks are unnecessary.

@martypdx Would it be a massive nuisance to merge this in, given #814? I think there's only one module that they both touch = `src/Ractive/initialise.js`.