try to add board to answer exercises
Merge branch 'develop' into board
Merge branch 'develop' into board
Merge branch 'develop' into board
try to use erb to do the board
Merge branch 'develop' into board
Merge branch 'develop' into board
Merge branch 'board' of github.com:wenhuizhao/kecheng into board
make the canvas can draw and input word together
make math questions can support words and lines successully
fix 190, 172
merge board
fix 187