plugin: use document.createElement instead of new Image()
plugin: use document.createElement instead of new Image()

Chrome 27+ has a bug where constructors don't work in the content
scripts. Workaround it by calling document.createElement() instead
of explicit constructors.

https://code.google.com/p/chromium/issues/detail?id=245296

em-/desktop-notifications-browser-extension,em-/desktop-notifications-browser-extension,em-/desktop-notifications-browser-extension,em-/desktop-notifications-browser-extension