make sure example plugins have all needed plugin json fields set

plugins/ExamplePlugin/plugin.json

@@ -12,5 +12,8 @@
       "email": "",
       "homepage": ""
     }
-  ]
+  ],
+  "homepage": "",
+  "license": "",
+  "keywords": []
 }
\ No newline at end of file

plugins/ExampleTheme/plugin.json

@@ -3,5 +3,15 @@
     "description": "Piwik Platform showcase: example of how to create a simple Theme.",
     "version": "0.1.0",
     "theme": true,
-    "stylesheet": "stylesheets/theme.less"
+    "stylesheet": "stylesheets/theme.less",
+    "homepage": "",
+    "license": "",
+    "keywords": [],
+    "authors": [
+        {
+            "name": "",
+            "email": "",
+            "homepage": ""
+        }
+    ]
 }
\ No newline at end of file