Default first arg where it can be None and was not previously

The goal here is to change all calls to methods of the form `add_cred(None)`
to `add_cred()` for convenience and clarity.

It has been verified that `acquire_cred_from()` can take nulls.

This closes pythongssapi#29.

gssapi/raw/creds.pyx

@@ -80,7 +80,7 @@ cdef class Creds:
             self.raw_creds = NULL
 
 
-def acquire_cred(Name name, lifetime=None, mechs=None, usage='both'):
+def acquire_cred(Name name=None, lifetime=None, mechs=None, usage='both'):
     """
     Get GSSAPI credentials for the given name and mechanisms.
 

gssapi/raw/ext_cred_store.pyx

@@ -92,9 +92,7 @@ cdef void c_free_key_value_set(gss_key_value_set_desc *kvset):
     free(kvset)
 
 
-# TODO(directxman12): some of these probably need a "not null",
-#                     but that's not clear from the wiki page
-def acquire_cred_from(dict store, Name name, lifetime=None,
+def acquire_cred_from(dict store=None, Name name=None, lifetime=None,
                       mechs=None, usage='both'):
     """Acquire credentials from the given store
 

gssapi/raw/names.pyx

@@ -163,7 +163,7 @@ def display_name(Name name not None, name_type=True):
         raise GSSError(maj_stat, min_stat)
 
 
-def compare_name(Name name1, Name name2):
+def compare_name(Name name1=None, Name name2=None):
     """
     Check two GSSAPI names to see if they are the same.