* added SK mock graphic for game menu screen
 * added special characters for SK language

Feature assetsdownloader

See merge request !45

#31 Boxes actions and minor glitches

Funkční verze střelnice. Zatím bez ukládání stavu.

See merge request !44

 * gradle task for uploading SNAPSHOT assets package
 * using SNAPSHOT assets package only in devel build type and DEV versions

 * reaction to download errors
 * error dialog on loading screen
 * retry of loading
 * internal dialog assets (external are not loaded)

 * calculate assets zip files checksum and save to server
 * download zip file checksum
 * check of downloaded assets zip file integrity