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/pushprocessor.js

@@ -80,7 +80,7 @@ const DEFAULT_OVERRIDE_RULES = [
  * @constructor
  * @param {Object} client The Matrix client object to use
  */
-function PushProcessor(client) {
+export function PushProcessor(client) {
     const cachedGlobToRegex = {
         // $glob: RegExp,
     };
@@ -503,6 +503,4 @@ PushProcessor.rewriteDefaultRules = function(incomingRules) {
  * noise.
  */
 
-/** The PushProcessor class. */
-module.exports = PushProcessor;