fix(HTMLImports): HTMLImportsLoaded event dispatch throws error on IE

When using a  script loader such as require.js or system.js the HTMLImports polyfill will not load correctly on all versions of IE. It will throw a JavaScript error causing the program to crash. This is caused by the use of `CustomEvent` to dispatch the HTMLImportsLoaded event, which results in `Object doesn't support this function`. By explicitly using the older `createEvent` technique, the bug is fixed.
should this be initCustomEvent? seems to have been supported since IE9 http://msdn.microsoft.com/en-us/library/ie/ff975299(v=vs.85).aspx
Though i'm not sure it really makes a difference.

also trivial comment, the quote style changes from single quote to double quote between those two lines :)