Changed default image alt attribute to be empty instead of filename

What do you guys think of this? I can't really see the benefit of setting the alt to the filename.

Here's an example that was bugging me in my app when the image didn't exist.

![screen shot 2013-07-11 at 6 59 25 pm](https://f.cloud.github.com/assets/207171/785633/67723206-ea88-11e2-806c-66fa2a30b284.png)