Adding bottom reached events + fix start event flood on bottom limit reached

[thochra]

Hello,

• start event is now triggered only once when the real start begins. It was flooding when bottom limit is reached and the top css property have to be updated to push up the sticked element.
• To keep the event triggering when top is updated after start, the update event have been added. So it floods like start did before.
• bottom-reached is triggered when sticked element reached the bottom space limit
• bottom-unreached is triggered when sticked element is not reaching the bottom space limit

Hope you will find this usefull...

[thochra]

Made my pull request from my master branch. Closing PR as other changes are coming into this pull request