Add configuration option to use credentials in all calls


This adds some overhead per-call, but less than calling
AuthenticateUser prior to every single call, since the 
authenticator token returned by the call is discarded.

Additionally, this allows a single stamps.com account to be
used from multiple processes simultaneously. Calling 
AuthenticateUser prior to each actual call results in 
frequent "conversation out of sync" and "invalid 
conversation token" errors, since the authenticator token
returned by AuthenticateUser may be invalid by the time
the subsequent call is made.