Feat/add rest and transitionstart events

[looeee]

This PR adds the new rest event and corresponding restEpsilon parameter discussed in #206.

Also added a new rest-and-sleep example to show the difference between the two events, and updated the readme.

While making the example, I discovered there was no way to reliably disable the buttons when starting a new transition using one of the buttons (which call .rotateTo and .dollyTo) - so I added an additional event, transitionstart. This fires whenever any kind of transition is started, which can be either a user input or a method with enableTransition = true.

I originally wanted transitionstart to fire only for a method with enableTransition = true, but not for user control. However, internally user controls use methods such as .rotate so there's currently no way to differentiate these. Probably that's ok though.

I checked in dist/ by mistake, let me know if you want me to remove it to make the code easier to review.

[yomotsu]

Thanks for the PR! I'll take a look.

I should have indicated that it's better to ignore the dist folder...but it's okay.

[yomotsu]

Thanks! Just merged!
I'll publish this change to NPM this weekend.