change application icon

I would like to thank Chris Ward (@tenzerothree, http://tenzerothree.com/) for
working on the art for these icons and bringing some eye candy to the project.

The PSDs made by Chris are available on our Dropbox [1], along with the exports
I made to create OSX and Windows icons. The PSDs are almost completly vector
and all the resolutions look really similar, except the 16px favicon which was
handcrafted to look better and more recognizeable on the smaller pixel budget.

For Mac OS X the icons were created using iconutils on the PNGs iconsets
exported from the PSDs. These even support retina resolutions (except 512@2x).

For Windows the .ico file was created with imagemagick. The included images
are 16px, 24px, 32px, 48px 64px, 256px. These are the resolutions listed on
MSDN for supporting Windows XP [2] and Windows versions based on Aero [3].
Only 32bit PNGs were used since it is 2013.

For Linux nothing changed yet, even though @wm4 talked about using the PNGs
directly there. This will probably be dealt with in a later commit.

  [1]: https://www.dropbox.com/sh/yelfoj9tbft7o06/A8vOT6JKaG
  [2]: http://msdn.microsoft.com/en-us/library/ms997636.aspx
  [3]: http://msdn.microsoft.com/en-us/library/aa511280.aspx