Template gateway

This is a template file that is used to create the base files for new gateway integrations using the [`gateway_generator`](https://github.com/Shopify/active_merchant/blob/master/generators/gateway/gateway_generator.rb).

We use [thor](https://github.com/erikhuda/thor) to run the generator command, which eventually calls into [`template`](https://github.com/erikhuda/thor/blob/master/lib/thor/actions/file_manipulation.rb#L108-L120) which does in fact use ERB to render the new files. As such, we need the ERB tags to populate the name of the gateway correctly.