More SBT build file refactorings.

Removed unnecessary global variables, added extra POM information,
removed redundant POM backup file.
Added sbt-sonatype plugin to simplify Facsimile releases.

Facsimile artifacts are published to the Sonatype OSS repository, which
is synchronized with the Maven Central repository.
Refactored again to support Sonatype publishing.
Added unified documentation plugin.

Further refactored project build to simplify publication.