Adds support for BillingInfo tokens

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Updates UpdateBillingInfoWithToken test to expect a 404

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Error; handle more error details

404s in particular were not really supported before
Merge pull request #24 from recurly/tokens

Adds support for BillingInfo tokens