Motivation:
The are many reasons why we should sign our artifacts. The main two are:

  - allow library users to verify who have release the jars
  - maven central accepts only signed artifacts (and this is the goal)

Modification:
Add profile that will be activated by maven release plugin or manually
with `-P sign-artifacts` option.

Result:
produced artifacts will be released.

Acked-by: Jürgen Starek
Target: master, 0.19