fix #6 to remove apis

  - canvas.char
  - canvas.text
  - canvas.paint
fix #7 to rename api

  - canvas.dot -> canvas.point
  - canvas.unmask -> canvas.noMask
change range index of custom tiles
add Canvas#triangle #8
add Canvas#quad #8
add Canvas#arc #8

  - but not be implemented yet
add Canvas#clip #8
fix Canvas#point
update examples
update readme
v0.2.1