Support for private github repositories

Use authenticated URI (when auth is set) to communicate with github so that private repositories may be jspm installed.

@guybedford

Fixes jspm-cli#71 
This is now using an api.github.com request because a simple GET request to github.com private repositories does not respond in a good way. The [api](https://developer.github.com/v3/repos/) request will be more robust.