Set up publication of PR final artifacts to artifactory-online.

This setup is dependent on a $typesafeArtifactoryPasswe environment
variable being passed at build ot builders, using the Jenkins Mask
Passwords plugin:

https://wiki.jenkins-ci.org/display/JENKINS/Mask+Passwords+Plugin
Merge pull request #1 from huitseeker/artifactory-publication

Publish snapshots of PR validation artifacts to typesafe's artifactory.