added support of attributes

Now you can add attributes to elements of configuration like this:
``` java
        executeMojo(
            plugin(
                groupId("org.apache.maven.plugins"),
                artifactId("maven-dependency-plugin"),
                version("2.0"),
                dependencies(
                    dependency("org.apache.maven.plugins", "some-plugin", "1.0")
                )
            ),
            goal("copy-dependencies"),
            configuration(
                element(
                    name("outputDirectory"),
                    attributes(
                        attribute("dir", "${project.build.directory}/foo"),
                        attribute("force", "true")
                    )
                )
            ),
            executionEnvironment(
                project,
                session,
                pluginManager
            )
        );
```