Add Polyfills For Window.atob and Window.btoa

**Ultimate Problem**
Consumers of `wf-common` need to encode and decode to/from base64. The `atob` and `btoa` functions are not available in IE9, so a polyfill needs to be added.

**Solution**
A polyfill was added in Compatability.js. The encoding/decoding logic was taken from https://github.com/davidchambers/Base64.js.

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Please review: @brianreed-wf @joshschlick-wf @timmccall-wf