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.
What happens if `patterns[i]` contains an invalid RegExp? Will it crash? Should we put the exec into a `try {} catch`, maybe?

I'd like to be able to unit test this "matching" mechanism, maybe extracting the regexp "precompiler" (lines 15-27) as a small little module than could be tested... but I would do it, don't worry :)