All difficulties available, added tutorial

Here is a new update on the game. Now, Medium and Hard Difficulty is now available. I also noticed a lot of people who played this game has no clue about how to play it, so I put a small tutorial on the menu screen to understand.

However, there is still a bug issue when clicking the back button in the credits and tutorial. I haven't found the solution, but this is as best as I can. Hope you enjoy :)