adjust width of the path button

因为按钮太窄的时候不太好点, 所以设置下最小宽度

before
![20141023a](https://cloud.githubusercontent.com/assets/760097/4750104/8dbbedae-5a8e-11e4-9915-a51b07187abc.png)

after
![20141023b](https://cloud.githubusercontent.com/assets/760097/4750106/9385cbce-5a8e-11e4-8959-13e045960899.png)