[B+C] Add method for setting explosion range to creepers.

**Justification for this:**

The Creeper has a field called explosionRange, but it isn't used by any Bukkit method. 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: coming soon
Bukkit: https://github.com/Bukkit/Bukkit/pull/866