Ported EventManager blog post to docs

- Original URI: http://www.mwop.net/blog/266-Using-the-ZF2-EventManager.html
- Edited and expanded to follow current API
Fix code-blocks

- Removed :startinline: and :number-lines: directives, and replaced with
  :linenos: directive
Merge pull request #824 from weierophinney/feature/eventmanager-tutorial

Port EventManager blog post to docs