Don't allow prisoners to be jailed into the same cell as someone else.

This will prevent overriding inventories in cell chests. When we tell
them the destination cell already has a prisoner in it, we try to find
the first empty cell in that jail but if all the jail's cells are full
then we tell them that as well.

src/main/java/com/graywolf336/jail/beans/Cell.java

@@ -42,6 +42,7 @@ public class Cell {
 		return this.p;
 	}
 	
+	/** Nullifies the prisoner data. */
 	public void removePrisoner() {
 		this.p = null;
 	}