Implement unit tests and integration test

[sam-wmd]

Currently, no tests have been implemented.
The goal is to write integration tests that will run the plugin commands for the goal:generate and check if the desired classes have been generated successfully.

• Implement a Test class for GenexMojo based on AbstractMojoTestCase class that runs the genex:generate command and generates files into a TemporaryFolder
• Check if all the desired files exist
• Check if the files content is coherent with the respective Velocity templates inside /src/main/java/template

maven-plugin-testing-harness documentation
[AbstractMojoTestCase](http://www.java2s.com/example/java-src/pkg/info/debatty/sparkpackages/maven/plugin/mojotestcase-ebcbd.html