+ Moved ee_putstr() and ee_putchar() in here.
  * libee/ee.h:
    + Got rid of ee_goto().
    + Moved <slang.h> or <curses.h> into libee.
  * Replaced ee_goto()/ee_putstr() pairs with ee_putstr().
  * Ditto for ee_putchar().

+ Added code comments.

+ ee_draw_thin_line().
  * test/demo.c:
    + Added a thin lines demo.

+ Removed draw_circle, use ee_draw_circle instead.
  * libee/circle.c:
    + New ee_draw_circle function.
  * libee/line.c:
    + Changed an internal function's name.