Convert src to ES6

The bulk of this is just export/import changes, though there's a couple pieces to highlight:
* We no longer use default exports. This is because it's discouraged by the JS community, though not in any official capacity.
* We now use `polyfillSuper` for some prototype inheritance because the tests, and sometimes webpack, break on "cannot call EncryptionAlgorithm without 'new'". It's very much a workaround, and definitely not needed when we use real classes.

There is some import shuffling to help keep the imports clean - this was done by my IDE.

src/ReEmitter.js

@@ -20,7 +20,7 @@ limitations under the License.
  * @module
  */
 
-export default class Reemitter {
+export class ReEmitter {
     constructor(target) {
         this.target = target;