Add library

Add gdx-tool.jar
Update Constants class

Add description file of the texture atlas,
Change VIEWPORT_WIDTH and VIEWPORT_HEIGHT
Add background image
Create Assets class
Update Main class of StrikeMouse-desktop

Add code to rebuild the texture atlas every time the game is run of
desktop.
Update StrikeMouseMain class

Add code to load, reload, unload the assets
Merge pull request #2 from lamnt570/NguyenTungLam

Nguyen tung lam