Merge branch '3-improve-skins' into 'master'

android/gradle.properties

assets/icons/level_easy.png

assets/icons/level_hard.png

assets/icons/level_medium.png

assets/icons/level_nightmare.png

assets/icons/level_normal.png

assets/icons/skin_default.png

assets/icons/skin_images.png

assets/icons/skin_retro.png

assets/icons/skin_symbols.png

assets/skins/default_1.png

assets/skins/default_2.png

assets/skins/default_3.png

assets/skins/default_4.png

assets/skins/default_5.png

assets/skins/default_6.png

assets/skins/default_7.png

assets/skins/default_8.png

assets/skins/retro_0.png

assets/skins/retro_1.png