Adding plugins block test and updating publishing plugins

[mattbsox]

Applying the plugin through plugins block requires adding the liberty-ant and liberty-common dependencies to the buildscript classpath. Gradle does not add runtime dependencies to the buildscript classpath when the plugin is applied with the plugins block. Addresses #441

• Added subproject to publish plugin to a test repo with the Gradle plugin repo coordinates
• Added a test using the plugins block to apply the plugin from the test repo
• Updated maven plugin to maven-publish plugin

An additional solution would be to publish a fat plugin jar containing the ant and common dependencies to the Gradle plugins repo.