Issue #1 Maven publishing using maven plugin

* Replace maven-publish plugin with maven plugin as signing does not seem to be supported in the former
  * `gradlew install` is back for local maven deploy
  * Publishing to sonatype requires `sonatypeUser` and `sonatypePwd` to be overidden on command line or in `~/.gradle/gradle.properties`
* Resolves #11
* Fixed missing property issue