Script returns 0 on Lint failure

Hi guys,

Thanks a lot for putting this tool together. Just have one small issue with it. The `html_lint.py` script appears to return 0 on Lint failure, making it difficult to use for automated build tools like Travis. It should return a non-0 code on Lint failure, I reckon.

```
> html_lint.py bad.html
1:1: Error: Indentation: Do not use tabs: Remove the tabs.
> echo $?
0
```