Classify: Provide an icon for the Invert action.

* app/src/main/res/drawable-svg-originals/ic_action_invert.svg:
  Created in Inkscape, based on the icon used in the Galaxy Zoo web app:
  http://static.zooniverse.org/www.galaxyzoo.org/images/icons.png
* app/src/main/res/drawable-*/ic_action_invert.png:
  Created with the Android Asset Studio with the default settings
  for action icons, with the Holo Dark theme:
  http://romannurik.github.io/AndroidAssetStudio/icons-actionbar.html#source.type=image&source.space.trim=1&source.space.pad=0&name=ic_action_icon_invert&theme=dark&color=33b5e5%2C60