2.6.0 Release

Background image support and misc fixes

• Can now set background images on notifications.
  • Add a drawable named onesignal_bgimage_default_image to use for all notifications.
  • Set onesignal_bgimage_notif_title_color and onesignal_bgimage_notif_body_color in your values/colors.xml to set text colors to match your image.
• Added additional checks for Google Play services.