[C + B] Add PigSaddleEvent. Adds BUKKIT-819

#### The Issue:
Currently, no event is fired when a pig is saddled.

#### Justification for this PR:
This is similar to the VehicleCreateEvent, and also represents a legitimate player event that plugins may or may not want to monitor.

#### PR Breakdown:
This PR implements a new cancellable PigSaddleEvent that is fired when a pig is saddled.

#### Relevant PR(s):
https://github.com/Bukkit/CraftBukkit/pull/1120

#### JIRA Ticket:
BUKKIT-819 - https://bukkit.atlassian.net/browse/BUKKIT-819