Add a parameter to force using the master KDC.

Issue #60.
Fix Kerberos salt encoding.

It's an array of enctype information, not a single one.
On AS-REQ error, fall back to the master KDC.

The user may have changed their password in the meantime. Make sure to
pick up the newest one. This is especially important for the "please
change your password" error when DES is removed. Closes #60.