Add ability to set the class of a field

The fieldtypes `text`, `password` and `textarea` always get a class of `large-text`, but there are multiple occasions where one might not need such a big input field.
So this pull requests adds the ability to set the class on a field, if no class is set, it will default to `large-text` as usual.

So now you're setting pages can look like this:
![screen1](https://cloud.githubusercontent.com/assets/1297781/5010396/9f8007ca-6a72-11e4-956a-deb34123571a.jpg)

Compared to how they look now:
![screen2](https://cloud.githubusercontent.com/assets/1297781/5010398/abe08648-6a72-11e4-8257-441b58cb4103.jpg)