Initial support for Groups

Create separate object for Users and Groups (using same base class)
Check for uniqueness before adding new users and groups
Remove user_container from everything but add operations
Abstract out a number of functions that are common across users and groups
Make sure all strings passed in to be in a filter are checked
Add new error message: No modifications specified

ipa-python/ipaerror.py

@@ -115,6 +115,11 @@ LDAP_MISSING_DN = gen_error_code(
         0x0005,
         "Entry missing dn")
 
+LDAP_EMPTY_MODLIST = gen_error_code(
+        LDAP_CATEGORY,
+        0x0006,
+        "No modifications to be performed")
+
 #
 # Input errors  (sample - replace me)
 #