redis: New destination driver

Based on the work of Tihamer Petrovics during the Google Summer of Code
2013, this driver implements a new destination driver that allows
syslog-ng to send events to a Redis key-value sture.

destination d_redis {
 redis(
  host("localhost")
  port(6369)

  command("HINCRBY", "hosts", "$HOST", "1")
 );
};

The driver requires the hiredis library, and will try to detect it
during configure-time.

Signed-off-by: Tihamer Petrovics <tihameri@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
.travis.yml: Install libhiredis-dev too.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>