Markdown should use local links for images

[garrettr]

Currently all of the links in encryption_works.md point to raw.github.com, which is a weird way to do it. I suspect this was done to get Github's auto-generated Markdown to work automatically. We should be able to instead use local links, e.g.

![](images/torbrowser.png)

instead of

![](https://raw.github.com/micahflee/encryption-works/master/images/torbrowser.png)

This is important because it breaks PDF rendering with Pandoc. It will also create problems as we edit the document and add new images.