Added start/end events for Arrow

[sudocode]

This will allow a developer to implement hooks for setting up and tearing down resources (such as VM allocation, etc).

var arrow = new Arrow;
arrow.on('start', function() { alert('started'); });
arrow.on('end', function() { alert('ended'); });
arrow.run();

[pranavparikh]

@sudocode ,
Thanks for the PR.
How do you plan to implement hooks ? Do you have an example ?

[pranavparikh]

@sudocode As we discussed last, the idea is to allow using Arrow as a library. I'm will test it a bit before taking in the PR.