Add submit method definition to form-data

This method is currently missing. Here is just a basic interface to pass type checks. If you'd help me figure out how to depend on Node TSD, I'd try to make signature more correct, and return http or https response instead of just `any`. Also planning to add `params` object interface.

form-data/form-data.d.ts

@@ -11,5 +11,6 @@ declare module "form-data" {
 		getHeaders(): Object;
 		// TODO expand pipe
 		pipe(to: any): any;
+		submit(params: string|Object, callback: (error: any, response: any) => void): any;
 	}
 }