RAQ : Allow referral code to be supplied with all API calls.
Updated documentation.
By setting the type appropriately for the client, IDE's can use static analysis to help with autocompletion.
Revert "RAQ : Allow referral code to be supplied with all API calls."

This reverts commit 02a6c20839372d31c050c633be9f100442ba59e8.
Merge pull request #28 from gocardless/pr/27

Fixed pull request for @RQuadling's comments