Use Markdown syntax rather than HTML for image links.

At some point pub.dartlang.org will support literal <a> and <img> tags, but
using Markdown links will get the readme displayed correctly for now.
Merge pull request #28 from nex3/readme-images

Use Markdown syntax rather than HTML for image links.
Fix WebGLMaterial wireframe line properties
Merge branch 'master' of https://github.com/threeDart/three.dart into libv2_cleanup