Site plugin

I wrote a plugin as suggested in this [tmc ticket](http://wald.intevation.org/tracker/index.php?func=detail&aid=6298&group_id=27&atid=303).

It integrates into the site as a project report, add the following snippet to the reporting section in a pom to see it in action:

```xml
  <reporting>
    <plugins>
...
      <plugin>
        <groupId>org.deegree</groupId>
        <artifactId>deegree-maven-plugin</artifactId>
        <version>1.18-SNAPSHOT</version>
      </plugin>
     </plugins>
   </reporting>

```

Once this pull request is accepted, I suggest to do a proper maven release for the plugin, then add this to the deegree parent pom.