[B+C] Add EntityBornEvent. Adds BUKKIT-3391

##### The Issue:
Currently there is no event to determine when two Ageable entities breed, thus no way to modify the experience gained in this event nor control over the parents and the child entities.
 
##### Justification for this PR:
Plugin authors should be able to modify how much experience is gained when breeding and cancel breeding attempts.

##### PR Breakdown:
This pull request adds a new event which is called when two Ageable entities breed.
 
#### Testing Results and Materials:
Testing was successful, first breeding attempt resulting in a baby piggy, second breeding attempt was cancelled and no baby pig was spawned.
[source](http://pastie.org/private/xlusrr6ndegzqykr2nq) || [compiled](https://dl.dropboxusercontent.com/u/29178507/Dev/EntityBornEventTest.jar) || [server log](http://pastie.org/private/tlrlzlkv1mzqwuicjx8uq)

##### Relevant PR(s): 
B-909 - https://github.com/Bukkit/Bukkit/pull/909 - Accompanying PR  
CB-1209 - https://github.com/Bukkit/CraftBukkit/pull/1209 - Accompanying PR

This pull request was originally submitted by @andrepl
https://github.com/Bukkit/Bukkit/pull/791 https://github.com/Bukkit/CraftBukkit/pull/1053
 
##### JIRA Ticket: 
BUKKIT-3391 - https://bukkit.atlassian.net/browse/BUKKIT-3391