adjust the button's background image and its size

## The button's background image has some problems with its top corners
### before
![ori](https://f.cloud.github.com/assets/993797/466219/285535ae-b654-11e2-8fc7-8bc5393996f0.png)

### after
![mod](https://f.cloud.github.com/assets/993797/466217/27d88798-b654-11e2-9e6e-031cb6ddef70.png)

## The button's size is not resized with its initial status
### before 
![ori2](https://f.cloud.github.com/assets/993797/466220/285e71f0-b654-11e2-9474-af7a6f6b2576.png)

### after (add `resizeToFitLabelText` to the button's title)
![mod2](https://f.cloud.github.com/assets/993797/466218/281ee486-b654-11e2-92d5-47866d38fa61.png)