Apparently the format of vpopmaild responses has been expanded and the responses
are conditional.
* Replaced the 'sometimes works' eq comparison with a regexp that always works.
* added tests for all 3 vpopmail plugins
* added cram-md5 auth support to auth_vpopmaild.
added auth_vpopmail plugin, using the perl-vpopmail module
added VPOPMAIL auth methods description to docs/authentication
added SEE ALSO section to each module, noting the VPOPMAIL description
Signed-off-by: Robert <rspier@pobox.com>