adds support for apex classes #26 (#29)

Currently, Salesforce developers cannot use CodeClimate despite CodeClimate having PMD as a plugin. This is because the PMD plugin filters out all files that are not Java (.java). This patch allows for the Apex class extension to be processed as well (.cls).

Related to #26

src/Config.groovy

@@ -78,7 +78,7 @@ class Config {
     def i = files.iterator()
     while(i.hasNext()) {
       def name = i.next()
-      if(!name.endsWith(".java")) {
+      if(!name.endsWith(".java") && !name.endsWith(".cls")) {
         i.remove()
       }
     }