[B+C] API for new mob spawner features. Adds BUKKIT-1998

##### The Issue:
There is no way for plugins to make use of the new spawner customisation that was introduced.

##### Justification for this PR:
Plugins may want to create custom spawners or modify the parameters of existing ones. I personally want to use this for dungeon generation.

##### PR Breakdown:
- Adds new methods to the CreatureSpawner interface to get and set the various fields.
- Adds support for all entity types that can be created using spawners to CreatureSpawner.setSpawnedType(EntityType entityType);
- Adds a method to set the properties of the spawned entity to those of an existing one, CreatureSpawner.setSpawnedEntity(Entity entity, boolean includePosition);
 
##### Testing Results and Materials:
Source - https://github.com/betterphp/SpawnerTest
Binary - http://ubuntuone.com/5bMZ7n3xCLxuw84on8sHDV

Provides 4 commands:
/gsp - Lists the current spawner properties
/ssp - Sets the spawner properties
/tnt - Makes a spawner than shoots TNT into the air
/fw - Makes a spawner that shoots fireworks into the air

All work on the spawner in the line of sight. A number of variations of spawner were created and their properties and behaviour checked before and after a server restart.

##### Relevant PR(s): 
B-849 - https://github.com/Bukkit/Bukkit/pull/849
 
##### JIRA Ticket: 
BUKKIT-1998 - https://bukkit.atlassian.net/browse/BUKKIT-1998