Refactor RuleSet creation

- RuleSet is now immutable
 - RuleSets are created through a RuleSetBuilder
 - RuleSetBuilder is accessed solely from RuleSetFactory
 - RuleSetFactory can now either parse XMLs for rule set creation,
    or create single rule rulesets

pmd-test/src/main/java/net/sourceforge/pmd/testframework/RuleTst.java

@@ -214,8 +214,7 @@ public abstract class RuleTst {
             ctx.setSourceCodeFilename("n/a");
             ctx.setLanguageVersion(languageVersion);
             ctx.setIgnoreExceptions(false);
-            RuleSet rules = new RuleSet();
-            rules.addRule(rule);
+            RuleSet rules = new RuleSetFactory().createSingleRuleRuleSet(rule);
             rules.start(ctx);
             p.getSourceCodeProcessor().processSourceCode(new StringReader(code), new RuleSets(rules), ctx);
             rules.end(ctx);

pmd-test/src/test/java/net/sourceforge/pmd/testframework/RuleTstTest.java

@@ -41,7 +41,8 @@ public class RuleTstTest {
         verify(rule).getMinimumLanguageVersion();
         verify(rule).getMaximumLanguageVersion();
         verify(rule).apply(anyList(), any(RuleContext.class));
-        verify(rule).getName();
+        verify(rule, times(2)).getName();
+        verify(rule).getPropertiesByPropertyDescriptor();
         verifyNoMoreInteractions(rule);
     }
 }