Removed deprecated singleton key in window creation

By default if you specify an id for the window, the window will only be created if another window with the same id doesn't already exist. If a window with the same id already exists that window is activated instead. 

See https://code.google.com/p/chromium/codesearch#chromium/src/chrome/common/extensions/api/app_window.idl&l=109
Merge pull request #31 from beaufortfrancois/patch-1

Removed deprecated singleton key in window creation