Added environment headers for cross-platform compatibility.

Changelog:
 1. Environment:
  - If the user is using Windows, it includes windows.h and does some
    awful coding with awful Windows.
  - Else, Hammertime supposes the user is using a UNIX machine.
  - Clearing the screen now has to be used with the macro CLEAR for
    cross-plaform compatibility.
 2. Screen Size:
  - It now calculates the console's size according to the user's OS.