Publish WWSK maven artifacts via Travis-CI automation

[emxsys]

Leverage Travis-CI to publish the WorldWind Server Kit maven artifacts to a public Maven repository.
Predicated on issues #38 (Travis-CI) and #39 (changing the maven group id).

See NASAWorldWind/WorldWindAndroid#105 regarding publishing artifacts to JCenter using the OSS JFrog Repository (https://oss.jfrog.org/).

• Publish to Bintray Publish WWSK artifacts to Bintray #53
• Publish from Bintray to JCenter Publish WWSK maven artifacts to JCenter #54
  Publish from JCenter to Maven Central Publish WWSK maven artifacts to Maven Central #55 (deferred from 0.3.0 milestone)
• Create Release Procedure documentation

[emxsys]

Merged pull request #41: generates the javadoc and sources jar files needed for Maven publication.