Use buffer position instead of screen position

It's bug when moving down or right there may be some errors if you enable the softwrap option.
For example if I have code like below, then I can't move below or right:
![motions_coffee_-__users_tony_github_vim-mode](https://f.cloud.github.com/assets/1253659/2355840/a0a21cd4-a5e1-11e3-91b2-6156db9d09dd.png)

btw, I notice that `#getCursorScreenPosition` is used at many places, maybe there're other errors, but I think we can fix them when we find the error.