Add support for multiple RuleSets

This adds the ability for the user to configure multiple rulesets with PMD as follows:

<reporting>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-pmd-plugin</artifactId>
<configuration>
<rulesets>
<ruleset>controversial</ruleset>
<ruleset>design</ruleset>
</rulesets>
</configuration>
</plugin>
</plugins>
</reporting>

It is assumed that the ruleset can be found in classpath:/rulesets/<name>.xml. This is the convention PMD uses and the user can add their own rulesets in a JAR and run them with the same configuration as long as they have a unique name.