print function for principal, principal.modify

dates are converted to iso8601 strings for printing the principal.

principals can now call “modify” to set multiple attributes and auto
commit the changes to the database all at once.

mod_name now properly includes the realm of the authenticated (default
realm used properly).
principal.modify changes
init now accepts db_args as a dict
kadmin.c error handling cleanup
added kdb errors, unified kdb, kadm, krb5 errors
cleaned up iterator functions
unit tests create keytab if needed.
unit tests generate test.keytab as needed

unit tests create the admin account and keytab for authentication using
an expect script. Utilizes configure principal name password and keytab
for generation
Merge pull request #16 from russjancewicz/development

Development