Truncate the directory to fit window size in the mode-line

Hi Pei,

This PR makes the `neotree` mode-line format fits in the current window size.

![capture d ecran 2014-12-31 a 17 44 07](https://cloud.githubusercontent.com/assets/1243537/5591041/c0fc22a8-9114-11e4-9a34-153fdc4e647b.png)

Cheers,
syl20bnr