Add SSL support.

This adds support for SSL connections to an SSL enabled job server.
To use SSL, three SSL-related files must be given when defining a
job server connection:

  keyfile - File with the private key
  certfile - File with the public key
  ca_certs - File with the CA public cert(s)

GearmanConnectionManager has been modified to simultaneously
support non-SSL and SSL connections alike. Nothing changes for
non-SSL connections:

  GearmanClient(['localhost:4730'])

SSL connections require that the host list use a dictionary to
define the connection:

  s1 = { 'host': 'localhost',
         'port': 4730,
         'keyfile': '/path/to/keyfile',
         'certfile': '/path/to/certfile',
         'ca_certs': '/path/to/ca_certs' }
  GearmanClient([s1])

Currently, all three SSL related files MUST be given and the port
must be specified in the dictionary.