Added autofree to c_create_mech_list

To make it easier to work with mech list OID sets,
a 'free' paramter (defaulting to True) has been added
to c_create_mech_list.  When set to True, gss_release_oid_set
will be called on the input OID set after converting it to
a list of mechanisms.
added implementation and tests for acquireCredImpersonateName -- see #31
added some things to type_wrappers to facilitate the s4u class development
fixed some bugs in type_wrappers.py
s4u support working...
fixed up some of the type wrapper code
s4u support working -- helper class in place, fixes to type wrappers to facilitate
added implementation and tests for addCredImpersonateName -- see #31