Added a cancelable PlayerMountEvent.

Hey, guys! I decided to add a cancelable PlayerMountEvent. This is fired whenever a player attempts to mount an entity, and can be applied to any entity that is eligible to be ridden (minecarts, boats, pigs, horses, modded entities, etc.) 

Demonstration - https://github.com/Enosphorous/PlayerMountEvent/blob/master/ExampleEventClass.java