Add task_successful_transfers to TransferClient
Add task_successful_transfers to TransferClient

Uses the new MARKER Paging Style to make this work.

Also minor fix to docstring on pause_info.
Resolves #87

sirosen/globus-sdk-python,aaschaer/globus-sdk-python,globus/globus-sdk-python,globusonline/globus-sdk-python,globus/globus-sdk-python