Improve paymentgateway admins

See #331 for screenshots and discussion. 

Requires #329 to be merged before this.

* make the core payment gateways look presentable in the WP fashion.
    * Chronopay
    * Google Wallet
    * PayPal Express Checkout 2.0
    * PayPal Payments Standard 2.0
    * PayPal Pro 2.0
    * Test Gateway