Add 'wpsc_gateway_name' filter.

For issue #673

Introduces a 'wpsc_gateway_name' filter so the display name of gateways can be customised on checkout page.

Gateways *should* have an 'internalname' defined.
I have used an implementation of the filter to recreate the current switch statement functionality, although I think this is just for legacy reasons?

I need a gateway itself could use this filter to change it's display text.