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 @joshschlick-wf @danielbank-wf @timmccall-wf 
This is okay I guess to simply inline. But would be simpler to extend and test if the individual polyfills were in their own files (tested separately) and simply imported and attached to the window here. This Compatibility function is becoming gross.