Compressing final sprite image via tinypng api

Since tinypng requires personal api key the only way to solve this problem was to set it via environment variable.
Just run `set TINYPNG_API_KEY=YOUR_API_KEY_HERE` for Windows or `export TINYPNG_API_KEY=YOUR_API_KEY_HERE` for Linux in your console and proceed with normal `grunt build`

Also, putting test.png file into source/sprites folder was the only way to check builds status