Add support for presenting azul3d.org/... packages.

- They use [semver](https://godoc.org/azul3d.org/semver.v1) for versioning, but all packages are currently hosted on GitHub.
- Once there are known custom domain names that also use [semver](https://godoc.org/azul3d.org/semver.v1), more general support can for semver can be added. Until then, this is for azul3d.org domain packages only.

Closes #24.

Example:

![image](https://cloud.githubusercontent.com/assets/1924134/5066107/178c5520-6de3-11e4-99c4-683cab9a127c.png)

/cc @slimsag