Added single_session argument for OC5 workaround

The OCS API in OC5 has trouble when reusing the same session.

This fix adds an argument single_session that can be set to False to
force reauthenticating for every call (not recommended).
Merge pull request #13 from PVince81/oc5-session-workaround

Added single_session argument for OC5 workaround