#949 Allow specification of artifact signing key.

Add :signing {:gpg-key "..."} option to project maps (global value)
and to :deploy-repositories maps (per-repo value). Does not yet work
in :repositories specs.

Also:
* Document per-repo :sign-releases option
* Delete unused defn deploy/signature-for
Remove outdated link to GitHub downloads page.
Tag tests which require network access.
Don't blow up when test eval-in-project doesn't return a number.

Hopefully only an issue when using :eval-in-leiningen.
Bump cheshire, slingshot, and pomegranate deps.