Upload JARs to Sonatype's OSS repo

The JAR itself, source and javadoc JARs are uploaded to Sonatype's OSS
repo. The current code should be available from the SNAPSHOT repo.
Add JAR signing to Gradle

JARs are signed before they are uloaded to a Maven repo.
Add Maven usage to README

There are SNAPSHOT JARs available via Maven. They should be used until
the first release. The documentation for this is in the README.
Merge pull request #1 from jehrhardt/sonatype-oss-repo

Add upload to Maven repository