Support gpg-agent as a gpg password alternative

[oleiade]

A comment on Hacker news mentioned that gpg agent would be a good alternative to password option and environment variable usage to set up the main gpg key password.

I've found this package from camlistore : https://github.com/bradfitz/camlistore/blob/b0d2a8f0e5f27bbfc025a96ec3c7896b42d198ed/pkg/misc/gpgagent/gpgagent.go implementing the gpg agent logic and communication.

Sounds like an easy and clever way to implement gpg-agent support in trousseau.

Ref #14 #12