Update icns file.

- Switch script to use rsvg instead of ImageMagick because ImageMagick
  was not properly converting the svg file on OS X.
- Add updated icns file with properly converted SVG
Add target to create dmg file on OS X.

Also add details on building a dmg.
Allow MacPlatformWindow to take an OS X window number.
Merge pull request #235 from rygwdn/misc-osx

Misc OS X fixes