datapkg_read() - allow github user/repo specification for path parameter

[sckott]

not sure if this makes sense, i don't know whether a lot of datasets will be on github or not, if not, then may not make sense

[danfowler]

This makes sense. There is currently a specification for resolving datapackage.json links from, say, GitHub URLs: http://specs.frictionlessdata.io/data-package-identifier/

This library should support that.