change user agent

differentiate ours from old 3rd party clients
Merge pull request #25 from recurly/user-agent

change user agent
fixed; creating accounts w/ address
Merge pull request #26 from recurly/account

fixed; creating accounts w/ address
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