Incident texts now support Markdown

Markdown support with [erusev/parsedown-extra](https://github.com/erusev/parsedown-extra).

You can use the `$incident->formattedMessage` attribute to get the formatted text of the message.

![screen shot 2014-12-30 at 10 33 13](https://cloud.githubusercontent.com/assets/246103/5577711/63c4d2f2-900f-11e4-84f5-3902884d4d78.png)