Add EventSource polyfill to support IE

[andrewhavens]

Evaluate and choose one of these popular polyfills to include:

• https://github.com/remy/polyfills/blob/master/EventSource.js
• http://github.com/rwldrn/jquery.eventsource
• https://github.com/Yaffle/EventSource/blob/master/eventsource.js

Here's a link that compares a few of them.