Create boilerplate for png-editor

Based upon mobile boilerplate
Upload and render png

This first GUI version implements file uploading and analysis of the
first chunk. File signature, png height and width are also read and
displayed.
Read and render chunks

The GUI renders all GUI for all the chunks.
Refactor: remove explicit new invokation

Style: new should not be called explicit. The object 'constructors' are
now plain functions that create an object with appropriate methods.
Add drag'n'drop support
Refactor PngImage to consistently use ArrayBuffer

Code refactor to carefully use ArrayBuffer instead of ByteArray to
leverage DataViews and other constructs. The png raw data is stored in
an ArrayBuffer. Changes through the interface change this ArrayBuffer.
The GUI elements should update to reflect this changes using listeners.
The edited image is also updated (by means of a listener).