New option: disable_sasl_mechanisms

The new "disable_sasl_mechanisms" option allows for restricting the list
of SASL mechanisms offered to the client.

Closes #339.
Merge pull request #340 from weiss/disable-mechanisms

New option: disable_sasl_mechanisms
Fix kick_user command
Merge pull request #341 from sezuan/fix-kick-user-command

Fix kick_user command