Update POM for Sonatype OSS; Update groupId

Modified the POM to inherit from the standard Sonatype OSS Parent POM.
The change also removes the <distribution> element, which isn't necessary
if we're inheriting from the Sonatype POM.

Added a release-sign-artifacts profile to automatically sign the artifacts
(required for Sonatype upload).

Changes the groupId from ch.qos.logback.extensions to org.logback-extensions.