Add GlobalCollect gateway

Add [GlobalCollect](http://www.globalcollect.com/) gateway.

This PR commits from [this branch](https://github.com/freerunningtech/active_merchant/commits/globalcollect-1.31).
`:billing_address` can interchangeably be passed in as `:address`. Ideally it would be defaulted such that it doesn't have to be required, too.