Add new engine component

TODO: Explain how an engine works. That it basically is a simply wrapper class that passes stuff from and to CC. Do heavily link to CC dev docs. Describe how engine components that are added to pmd might change the Groovy script. Come up with other cool ideas to extend PMD.

Beside extending the underlying PMD framework people could also add totally new mechanisms to evaluate code. Code would then not only run through PMD but also through other internal components. Just imagine using the Tooling Api and then reporting back on usused code across class boundaries as implemented by Salesforce.com MVP Andrew Fawcett.