Add a warning when compile a class that extends another class which is not include

[ybootin]

for exemple :

module mymodule {

export class myclass extends anotherclass {

should throw a warning because another class is not referenced as this in the header :

/// <reference path="./anotherclass.ts" />

this cause in error in code, because extends class is not includes in base class at runtime :

Uncaught TypeError: Cannot read property 'prototype' of undefined

[RyanCavanaugh]

Duplicate of #21