Automatically resize panel, like Chrome

Note: There are edge cases where height/width changes are not detected.
E.g. when you modify the width/height of the document with CSSOM
(stylesheet manipulation).

Because the dimensions are only checked after a DOM mutation, just
trigger the event, e.g. by using setAttribute.
+Test for hide/show panel

Includes a SDK module that serializes XML to HTML,
for the ease of debugging
Destroy popup when hidden
Trigger unload event on unload
Release 0.2.0