Added support for plugin dependencies

I added support for plugin dependencies.  I would appreciate it if you could take a look at it and see if this is something that you want to add to the mainline

Here's an example of what it looks like when you use it : 

        executeMojo(
                plugin(
                        groupId("org.apache.maven.plugins"),
                        artifactId("maven-assembly-plugin"),
                        version("2.3")
                ),
                goal("single"),
                configuration(
                        element(name("appendAssemblyId"), "false"),
                        element(name("finalName"), project.getArtifactId()),
                        element(name("descriptorRefs"),
                                element(name("descriptorRef"), "puppet-module"))
                ),
                dependencies(
                        dependency(
                                groupId("com.company.descriptors"),
                                artifactId("my-descriptor"),
                                version("0.1.0-SNAPSHOT")
                        )
                ),
                executionEnvironment(
                        project,
                        session,
                        pluginManager
                )
        );


I extended the dsl and made sure that it does not break backwards compatibility. All tests pass and i added one extra test that illustrates the feature