Update Dolphin logos

* Remove the Beta logo as Dolphin is now using the Android Market Beta
  Testing program, therefore, the logo is the same as the one for the
  stable version.

   - http://dolphin.com/beta/
   - https://support.google.com/googleplay/android-developer/answer/3131213?hl=en

* Rename `dolphin` to `dolphin-android` and create a new `dolphin-ios`
  directory as Dolphin for iOS has now a different logo from the one
  used on Android.