EnderSignalLaunchEvent - Implements BUKKIT-391

Added a new cancel-able event much like the current ProjectileLaunchEvent but for EnderSignals (which are not considered projectiles). Event is fired when an EnderSignal is added to the world (caused by a player right clicking anything but an end portal frame with an Eye of Ender in hand).

Implements:
https://bukkit.atlassian.net/browse/BUKKIT-391

Corresponding Bukkit PR:
https://github.com/Bukkit/Bukkit/pull/738

PS: This is my first pull request so comments and positive criticism greatly appreciated :)