[B+C] Added SpawnReason for creature spawned by nether portal. Fixes BUKKIT-3148

##### The Issue:
There is no clear way to detect when Pig Zombie (or other entity, if ever so) is spawned by Nether Portal.

##### Justification:
Allows developers to easier detect when entity is spawned by nether portal. Right now Bukkit returns spawn reason as spawned by using spawning egg when pig zombie is spawned by nether portal. This makes confused a lot of developers and requires finding a way around that problem.

##### PR Breakdown:
This PR is adding a clear way to check if entity was spawned by nether portal and fixes problem with wrong spawn reason when pig zombie is spawned by nether portal.

##### Testing Results and Materials:
[SpawnReasonTest plugin](http://www.mediafire.com/?mpr9xzgbxh1z8sp) - Prints EntityType and SpawnReason in console when CreatureSpawnEvent occurs.
[SpawnReasonTest plugin source code](https://github.com/KodekPL/SpawnReasonTest) - Source code of SpawnReasonTest plugin.
[Pig Zombie Grinder Map](http://www.mediafire.com/?i973jkyzcl3zlzj) - Map with pig zombie grinder makes checking spawn reason of pig zombie spawned by nether portal easier.

##### CraftBukkit PR:
Bukkit/CraftBukkit#####

##### JIRA Ticket: 
[JIRA Ticket #3148](https://bukkit.atlassian.net/browse/BUKKIT-3148)