ANNO-187 Added CustomEvent polyfill

__ULTIMATE PROBLEM:__
DOM4 Has added support for CustomEvent: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent

But it is not supported in IE. 
https://jira.webfilings.com/browse/ANNO-187

__HOW IT WAS FIXED:__
Added a polyfill constructor to support CustomEvent in IE.

__TESTING SUGGESTIONS:__
This was added for: https://github.com/WebFilings/wf-js-annotations/pull/250
You can follow the testing suggestions outline there.

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__FYA:__  @timmccall-wf @trentgrover-wf 
__FYI:__ @tonyhelvey-wf @ericdebusschere-wf 

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