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
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
Split S4U Code into Separate Module

This splits the S4U code into a separate module, and makes
it an optional import in gssapi.base, since it is not part
of RFC 2743.

To disable the compiling of the S4U code, simply remove the
module from the list of extension modules in setup.py
Merge pull request #33 from DirectXMan12/feature/s4u_support

merge feature/s4u_support -- closes #31