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

They use semver for versioning, but all packages are currently hosted
on GitHub.
Once there are known custom domain names that also use semver, more
general support can for semver can be added. Until then, this is for
azul3d.org domain packages only.
Minor refactor.

Be more consistent in variable naming.
Return more descriptive error.
Merge pull request #25 from shurcooL/feature/azul3d.org-support

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