Add PMD, for now with no rules

Unlike findbugs, PMD is very easy to configure. Should be very useful as
long as we set a correct ruleSet.

F-Droid/build.gradle

@@ -1,6 +1,7 @@
 apply plugin: 'com.android.application'
 apply plugin: 'witness'
 apply plugin: 'checkstyle'
+apply plugin: 'pmd'
 
 repositories {
     jcenter()
@@ -203,11 +204,24 @@ task checkstyle(type: Checkstyle) {
     configFile file("${project.rootDir}/config/checkstyle/checkstyle.xml")
     source 'src', 'test/src'
     include '**/*.java'
-    exclude '**/gen/**'
 
     classpath = files()
 }
 
+pmd {
+    toolVersion = '5.4.1'
+    consoleOutput = true
+}
+
+task pmd(type: Pmd, dependsOn: assembleDebug) {
+    ruleSets = [
+        //'java-basic',
+        //'java-unusedcode',
+    ]
+    source 'src'
+    include '**/org/fdroid/**/*.java'
+}
+
 // This person took the example code below from another blogpost online, however
 // I lost the reference to it:
 // http://stackoverflow.com/questions/23297562/gradle-javadoc-and-android-documentation