Added EntityBreedEvent, adds BUKKIT-3391

The EntityBreedEvent is fired when 2 Ageable entities breed.  This event allows for control over the amount of XP Dropped, as well as allowing direct access to both parents and the child entities.  It is also cancellable which will prevent the breeding.

Leaky: https://bukkit.atlassian.net/browse/BUKKIT-3391
CraftBukkit: https://github.com/Bukkit/CraftBukkit/pull/1053