Don't require spree_frontend and use absolute path for connect-dwolla button image

We shouldn't require the spree_frontend in the css because many stores use their own css but will still want make use of the css for the Dwolla payment method.

Also, in our app the path for the dwolla button was interpreted by the browser as `/assets/connect-dwolla.png` instead of `/assets/store/connect-dwolla.png`, the correct path. Using an absolute path corrects the issue.