allow to specify the Maven installation

This adds a new top-level method and a variant of the maven step which can be configured with a closure.

```groovy
job(type: Maven) {
    ...
    mavenInstallation('Maven 3.0.5')
    ...
}
```

```groovy
job {
    ...
    steps {
        ...
        maven {
            rootPOM('pom.xml')
            goals('clean install')
            mavenInstallation('Maven 3.0.5')
        }
        ...
    }
    ...
}
```