Added rscall as an easy way to create redstone callbacks

This adds a drone command in contrib that lets you set a redstone callback that is specific to the location of the Drone.  This allows something like "/js new Drone().rscall(function(event){console.log("My button was pressed!");});" and the console message will be displayed if and only if the block at the location of the Drone at that time has its redstone state changed.

(This is the same as the previous pull request, but now I've figured out github branching so contains no other extraneous changes.)