refactor. Cleaner way to handle method_missing

separate concerns for method_missing and respond_to_missing?
OptionSelectable multi should not be cleared by default

- CheckboxGroup and SelectList will not clear all first
  when you set an option
- respect the original Watir select_list.select
  which appends the option to already selected ones.
PageObject dispatches and delegates to lower element
fixed bug with resond_to after adding method_missing

- initialize_this relied on respond_to? for strategy
  that is no longer valid that we have respond_to_missing?
- change to check for actual object. This constraints us to
  implement Hash, Watir and PageObject rather than objects
  that merely respond to those messages.