Download oauth2-useragent JAR file with all dependencies

Provides classes to facilitate the implementation of "4.1. Authorization Code Grant" from RFC 6749, specifically by auto-detecting a suitable user-agent (and informing the user if any system requirements are unmet and preventing the use of a user-agent), launching the user-agent and directing it to the authorization endpoint, waiting for the results and returning either the authorization code or the reason for failure.