Fix scope bindings and track displayed month.

This PR is built on top of my work in #17, but is separate from it, so let me know if you want it on its own.

I've made changes so the scope variables bound from the parent scope will be reflected in the calendar. This allows for changing min/max/disabled dates. I've also changed the `default-date` option into `display-month` and it is updated in the parent scope (assuming the expression is assignable). If you change the `display-month` it will re-render the calendar.