[ProgressBar] OSX, Win, Ubuntu implementation

adding javascript function
Window.setProgressBar(double val)
valid values are 0 to 1 
val < 0 means remove the progress bar
val > 1 means indeterminate

Mac screenshot
![screen shot 2014-08-06 at 1 51 53 pm](https://cloud.githubusercontent.com/assets/4043527/3822859/f46023fa-1d2f-11e4-9994-57fcf45ecbfa.png)

Windows screenshot
![screen shot 2014-08-06 at 1 50 20 pm](https://cloud.githubusercontent.com/assets/4043527/3822860/f93a4464-1d2f-11e4-8f74-c8aa4176809e.png)

Linux screenshot (currently only works with Ubuntu)
if anyone know if these features are supported on Gnome / KDE, let me know
in Ubuntu you must make a .desktop file, then set the app env NW_DESKTOP to that .desktop file, or by default it will search for nw.desktop
![screen shot 2014-08-05 at 6 53 07 pm](https://cloud.githubusercontent.com/assets/4043527/3822861/febd2640-1d2f-11e4-9096-eb5a8451a916.png)