Styling - support for logos

Several users has asked for support for displaying card logos, and a overall better styling possibility for the module display in cart. Thus I added back the old config and support for it, after modifying it slightly.
As default it will match t.ex. the Klarna modules, but styling can easily be overridden through the stores stylesheet.

![skjermdump fra 2014-06-17 15 47 53](https://cloud.githubusercontent.com/assets/858132/3300163/ab1f8412-f61d-11e3-8c70-35be6f5e6d07.png)