BUG: style.bar results in incorrect gradient using specific browser
It's not a bug on pandas side, but CSS `linear-gradient` with `0%` width seems not be rendered properly using specific browsers.

Below is the output using Mac OSX 10.10.5 / Firefox. 

![2015-11-23 10 53 47](https://cloud.githubusercontent.com/assets/1696302/11329349/58e78d80-91de-11e5-82e6-6c90b80d93fb.png)

The fix simply exclude `linear-gradient` if its width is 0.

After the fix (same environment)

![2015-11-23 10 54 18](https://cloud.githubusercontent.com/assets/1696302/11329369/9d37dce2-91de-11e5-9636-29d2b2a843cd.png)
