Separates the publishing/signing of artifacts into new Tasks/flows from
standard publishing.  While this may be confusing to some, I think it
will help solve a lot of frustration/confusion in the long run.

Review/Use by @paulp and others.
Merge pull request #34 from sbt/wip/jsuereth-separate-sign-publish

Separates the publishing/signing of artifacts into new Tasks/flows from