Create nexus.rb

The Nexus forge is basically the same as the directory forge but modified to accommodate the Nexus file storage structure of Maven artefacts. It assumes that the nexus directory points to the groupId folder for finding puppet modules. 
Eg: 
Maven GAV: puppet-module:author-modulename-0.0.1-SNAPSHOT:(tar.gz,json)
Nexus Dir config: <base directory/puppet-module
Nexus path: author-modulename/0.0.1-SNAPSHOT/author-modulename-0.0.1-SNAPSHOT.(.tar.gz,json)