add rasterize.coffee to gem

The gem was missing the rasterize.coffee file.

Added script/rasterize.coffee to gem and changed the reference in
lib/dragonfly_phantomjs/phantom_js_encoder.rb. This was referring to
Modulor::Engine.root, but is now changed to DragonflyPhantomjs.root
which uses the added method self.root defined in
lib/dragonfly_phantomjs.rb.

Also updated readme by adding username to the gem url in the
Contributing section.
Merge branch 'add_script'
update
Merge branch 'add_script'