Selfify example (#251)

* Selfify example for how to access files article

* Restored selfify.js and updated choose_file.js as suggested by Luca

* Updated comments

* Updates for feedback from Will

* Updates from the second round of feedback

* removed activetab from manifest, removed document.body.appendChild(info);, renamed the content script (selfify > content), renamed the listener (selfify > injectImage) and moved example to imagify folder.

* Correctly renamed folder and update manifest.json

imagify/sidebar/sidebar.css

@@ -0,0 +1,15 @@
+html, body {
+  width:  100%;
+  height: 100%;  
+}
+
+#drop_zone {
+  border: 5px solid blue;
+  width:  100%;
+  height: 100%;
+}
+
+#drop_zone_label {
+  margin: 1em;
+  display: block;
+}