The changes included in this PR are as follows:

• Module is changed to a package
• mouse_cursor.bmp is included with package
• find_and_init_boot_mouse loads included bitmap (DEFAULT_CURSOR) if cursor_image is not provided
• If cursor_image=None, the following occurs:
  • BootMouse.tilegrid=None
  • The supervisor.runtime.display size is ignored
  • Two private variables, _x and _y, are used to keep track of mouse position
  • No boundaries are applied to mouse coordinates (can be controlled by user using x and y properties if desired)

Fixes #7