[B+C] Add a hurtEntities setter to FallingBlock. Adds BUKKIT-4752

##### The Issue:
There is no current way to set FallingBlock's hurtEntities field without accessing NMS.

##### Justification for this PR:
With this addition plugins will be able to set hurtEntities through Bukkit API.

##### PR Breakdown:
This PR adds a new method setHurtEntities to the FallingBlock interface in Bukkit, and implements that method in CraftFallingSand in CraftBukkit. Additionally it makes hurtEntities field public instead of private in EntityFallingSand.
 
##### Testing Results and Materials:
Test plugin code: https://gist.github.com/YaLTeR/6509037
Test plugin JAR: http://www.mediafire.com/?22hphez9qw2a423

Test plugin commands:
/hetester nohurt - spawns an anvil ontop of you that does not hurt you.
/hetester hurt - spawns an anvil ontop of you that does hurt you (made using the API method from this PR).

##### Relevant PR(s): 
 
B-#### - https://github.com/Bukkit/Bukkit/pull/#### - the new API method.
CB-#### - https://github.com/Bukkit/CraftBukkit/pull/#### - the new API method implementation.

##### JIRA Ticket: 
BUKKIT-4752 - https://bukkit.atlassian.net/browse/BUKKIT-4752