Added JSON messages and protocol version checking, see https://github.com/phetsims/joist/issues/176
Hook the iframe API into Sim.js through axon trigger, see https://github.com/phetsims/joist/issues/176
Fixed: setActive message wasn't being sent using the right type
Added an ability to make the sim inactive using the iframe API, see https://github.com/phetsims/joist/issues/176
Fixed lint issues
Rewrote PhetButton using PushButtonModel and PushButtonInteractionStateProperty, see #177
Merging in master