Use port 80 by default

[imobachgs]

The Agama web server uses port 3000 by default. But nowadays, there is no good reason for doing that. And, if you want, you can change the port with something else.

In the Agama live media, the web server listens on the standard port 80 but the CLI expects our "standard port" 3000.

We need to add an --api switch to the CLI, but let's do that in a separate PR.

[coveralls]

coverage: 69.889%. remained the same
when pulling 54ca0e0 on use-port-80
into 066d480 on master.