Better interaction with Vim jumps

Hi,

This pull request enables EasyMotion movements to trigger [jumps](http://vimdoc.sourceforge.net/htmldoc/motion.html#:jumps).

Currently, EasyMotion only sets the `'` mark without remembering the column the cursor was in before moving. With this update, the `'` mark is still set (now taking the cursor column into account). Moreover, these movements are now integrated into Vim's jump history: you can go back/forward to these cursor positions using `<c-o>` and `<c-i>`.