Update gm.d.ts

Supporting alternate constructors listed at https://github.com/aheckmann/gm#constructor

gm/gm.d.ts

@@ -9,6 +9,9 @@ declare module "gm" {
 	import stream = require('stream');
 
 	function m(image: string): m.State;
+	function m(stream:NodeJS.ReadableStream, image?: string): m.State;
+	function m(stream:Buffer, image?: string): m.State;
+	function m(width:number, height:number, color?:string): m.State;
 
 	namespace m {
 		export interface ClassOptions {