[B+C] Add LeadKnot entity. Adds BUKKIT-4570

The Issue:

There is no entity class in Bukkit for lead knots.

Justification for this PR:

Lead knots are entities created by player while right clicking a fence with at least one entity on a leash. In Bukkit there is no corresponding entity class.

PR Breakdown:

LeadKnot is extending Hanging. The name is choosen according to http://www.minecraftwiki.net/wiki/Data_values#Entity_IDs

Testing Results and Materials:

Working fine. Detailed material on request.

Relevant PR(s):

CB-1206 - https://github.com/Bukkit/CraftBukkit/pull/1206
B-897 - https://github.com/Bukkit/Bukkit/pull/897 (for leashing Entities)
CB-1190 - https://github.com/Bukkit/CraftBukkit/pull/1190 (for leashing Entities)

JIRA Ticket:

BUKKIT-4570 - https://bukkit.atlassian.net/browse/BUKKIT-4570
BUKKIT-4583 - https://bukkit.atlassian.net/browse/BUKKIT-4583 - partly related