Fix for issue #31 Link images

Uses the [node-ecstatic](https://github.com/jesusabdullah/node-ecstatic) middleware to serve static files (images, pdfs, etc) from the wiki directory. Markdown files (.md extension) are excluded so that wiki source is not exposed to client browsers.
I don't think we need to put a try/catch around the exec because it will return null if the whitelist pattern doesn't match the regexp pattern "/expression/flags' but I do need a try/catch block around the RegExp constructor in case of an illegal expression or bad flags. Good catch :+1: