Add EntitySaddleEvent. Adds BUKKIT-819

Split from GlowstoneMC/Glowkit#3
Original Bukkit PR: Bukkit/Bukkit#840
Additional changes: Added a `Player` reference to the event
Glowstone implementation: Not done

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Previously it was not possible to determine when an entity was saddled.
This commit changes that by adding the applicable event for the action.