Apply patch from http://codelabs.ru/patches/offlineimap/2013-add-SASL-PLAIN-authentication-method.diff by konvpalto
Make authentication mechanisms configurable

- added the configuration option "auth_mechanisms" to the config file

- if the auth_mechanism isn't given, all mechanisms will be tried, the
  current default is PLAIN,LOGIN,CRAM-MD5,GSSAPI

- a mechanism is only tried if it is enabled in authmechs