Merge pull request #138 from bvssvni/master

Fixed current directory bug in `load_image`
Merge pull request #142 from bvssvni/master

Updated example in slides
Added all SDL2 keycode to `keyboard::Key`
Merge pull request #146 from Coeuvre/master

Added all SDL2 keycode to `keyboard::Key`
Create tutorial.md
Added an examples to print 'keyboard::Key' when pressed a key
Print keycode when released a key
Merge pull request #147 from Coeuvre/master

Added an example to print 'keyboard::Key' when pressed a key
Added titles for each chapter in tutorial
Modified user input example to print mouse button events