Allows you to use OnInspectorGUI in MonoBehaviour directly, rather than having to create a CustomEditor

MonoBehaviourInspector

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;
using System.Reflection;

[CustomEditor (typeof(MonoBehaviour),true)]
[CanEditMultipleObjects]
public class MonoBehaviourInspector : Editor
{
	public MethodInfo inspectMeth;

	public void OnEnable()
	{
		inspectMeth = target.GetType().GetMethod("OnInspectorGUI",BindingFlags.Instance|BindingFlags.Public|BindingFlags.NonPublic);
	}

	override public void OnInspectorGUI()
	{
		DrawDefaultInspector();

		if(inspectMeth != null)
		{
			foreach(var eachTarget in targets)
			{
				inspectMeth.Invoke(eachTarget, new object[0]);
			}
		}
	}
}