[B+C] Add method for setting explosion range to creepers. Adds Bukkit-4171

**Justification for this:**

The Creeper has a field called explosionRange, but it isn't used by any Bukkit and CraftBukkit method.
In addition to that it isn't used to when the creeper explodes, this dismatches vanilla behaviour. (since updated to Minecraft 1.4(.2))
This PR adds a method to the Creeper which made this value accessible and changeable

**PR Breakdown:**

Added getExplosionRange and setExplosionRange(int) method to Creeper interface

JIRA: https://bukkit.atlassian.net/browse/BUKKIT-4171
Bukkit: https://github.com/Bukkit/Bukkit/pull/866