Fix javascript output alignment bug

This fixed the output alignment issue in the notebook for javascript cells. 

I forced the display attribute of the output_area to always be `flex`. That way, when you call `.show()` in a javascript cell, it won't inherit the default display attribute `block`. This was causing the issue shown here:

![screen shot 2013-08-23 at 11 18 36 pm](https://f.cloud.github.com/assets/2791223/1170492/a662fe14-20de-11e3-9770-fd76f5d95ae5.png)

It should be aligned underneath the cell like this:

![screen shot 2013-08-23 at 11 14 30 pm](https://f.cloud.github.com/assets/2791223/1170499/b16a253a-20de-11e3-9ed0-9c8211116a38.png)

Also, I added a CSS class for javascript output. This isn't used to fix this issue, but we have it for all other output types, so I added it for consistency. We may use it in the future.