Fixing a closing tag

<button> tag should be closed with </button>, not </a>
Small update to Introducing Events

Adding a link to "DRY" in case people are not familiar with that acronym.
Event Extensions: Use warning class instead of blockquote for note. Fixes #240.
Basic Plugin Creation: Fixed property reference in example. Fixes #364.
0.2.9
Merge remote-tracking branch 'publico/master'