Create the Shell class

[rijkvp]

Currently the players & enemies can't fire yet. To make them able to fire we need a Shell class wich represents the shells the tanks are able to fire.

• [x] Create a new class called 'Shell' & place it in a new processing tab
• [x] The class should have a x & y variable witch hold the position of the shell
• [x] Create a constructor with the x & y coordinate of the shell
• [x] Create a draw function witch draws the shell sprite