gpg-agent detection

[tschuy]

the current gpgagent "library" is a subpackage of an unrelated project. Currently, it's necessary to set the GPG_AGENT_INFO env variable when running on a system with gpg-agent 2.1 or above, since it no longer sets the variable, since the gpgagent library from camlistore doesn't understand how to find gpg-agent since the environment variable was deprecated: https://www.gnupg.org/faq/whats-new-in-2.1.html

[tschuy]

Rudimentary "detection" has been implemented in gotp.go: https://github.com/tschuy/gotp/blob/master/gotp.go#L21

I'm not particularly happy with it, so I'm leaving this open as a reminder to try to really get this fixed.