Turn sponsor plans into a more visible list, allows direct donations via Stripe forms

This is a work in progress, not ready for merging yet, but a basis for discussion.

This changes makes the plans more visible and a bit more distinctive:

![](http://s3itch.paperplanes.de/Sponsors__Rails_Girls_Summer_of_Code_20130525_090914.jpg)

It also adds the possibility of a direct donation via Stripe:

![](http://s3itch.paperplanes.de/Sponsors__Rails_Girls_Summer_of_Code_20130525_091205.jpg)

Needs an addition to support VAT as well, but that could be a smaller link below the "Become a Sponsor" link.

It's also missing the publishable Stripe token to be fully set up.