Adds token_id support to Recurly_BillingInfo

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Adds client argument to Account constructor

to support passing the mock client

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Adds a series of tests around BillingInfo token_ids and Account creation

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Merge pull request #83 from recurly/tokens

Adds token_id support to Recurly_BillingInfo