Add Sonatype OSS as parent to support pushing to Maven Central

Would love to see this artifact published to Maven Central for better accessibility. Sonatype OSS offers free artifact hosting for open source projects and syncs to Maven Central. See https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide for further required steps, but this should cover most of the pom changes. Note that you may have to change the groupId and add a license block to conform to their standards.

This pull requests targets https://github.com/sdouglass/spring-social-tumblr/issues/11