Add Android browser logo

Ref alrra/browser-logos#45
    alrra/browser-logos#55

Close alrra/browser-logos#55
Further losslessly optimize the Android logo
Add Opera Mini logo

Ref: http://www.opera.com/mobile/mini
Add `main-mobile.png`

Close alrra/browser-logos#54
      alrra/browser-logos#56
Make generate-images.sh create `main-mobile.png`

* Change the `generate-images.sh` script so that it automatically
  generates the `main-mobile.png` image when any of the logos included
  in it are changed.

* Regenerate the `main-mobile.png` image.

Ref alrra/browser-logos#54
    alrra/browser-logos#56