Automatically find gpg-agent socket path

[forsberg]

Adds code that uses the gpgconf command to find the gpg-agent socket path. The old behaviour is kept, it's only if the call to agent.Dial has the empty string as filename argument that the code will try to find the socket automatically.

Tested on Ubuntu bionic.

Also made a few minor changes to the README, and made travis builds work even on forks. In addition, created a script for running the tests locally.

Please point out any go-related beginners mistakes, it's my first go code :-)

[prep]

Hey Erik,

Apologies for the late reply and thank you for your PR 👍 If this is indeed your first time writing Go, then you're off to a great start!

I'd be happy to merge your PR, with the exception of runtests.sh script and the stuff that references it. I prefer to keep those kinds of scripts out of the repo, but if they're now required to run the tests I'd rather spend some more time on making the tests more sane to their environment, than to adjust the environment to insane requirements on the tests 😉