#170 Ensure checked has a default value of false if not set

[azariah001]

There is a known bug where when checked is bound to in a template value it's default value when unset is 'undefined' which results in the essential _compute functions not being called. Thus the hidden class (most importantly) is not defined and the default checkbox remains visible within the paper-checkbox location resulting in very bad UX.

This is probably a very inefficient method but.... it works. And doesn't loop so it's a start at least. And because it doesn't rely on passing itself into a computed function, it is still called when it's undefined and thus solves the primary problem indicated in #170