Add an entry to explain events

Specifically, event listeners in the browser
Use singular entry title: event, not events
Edit entry on events given @zspencer's feedback

Discuss events as a more abstract concept, and provide example of
publish/subscribe model
Reference callback entry, not callbacks (plural)
Add JS example for event definition
Use camelcase for javascript example
Merge pull request #2 from codeunion/add-entry-on-events

Add an entry to explain events