src/Session.js: Replace 'referred' event with 'refer' handler helper

fixes #48

To migrate applications, replace:

  session.on('referred', referFollowed);

with:

  session.on('refer', session.followRefer(referFollowed));
Merge pull request #49 from joseph-onsip/48

src/Session.js: Replace 'referred' event with 'refer' handler helper