Add redis plugin.

Add redis plugin.

This commit adds a new redis plugin, which connect to a number of redis
server and get information about their status, using the libcredis > 0.2.2
library. The plugin get the following information:

* bgsave_in_progress
* change_since_last_save
* connected_clients
* connected_slaves
* total_command_processed
* total_connection_received
* uptime_in_seconds
* used_memory

Here is an example of configuration file:

<Plugin redis>
	<Node example>
		Host "localhost"
		Timeout 2000
	</Node>
</Plugin>

And the output of rrdtool plugin must be something similar to this:

redis-example/bgsave_in_progress.rrd
redis-example/changes_since_last_save.rrd
redis-example/connected_clients.rrd
redis-example/connected_slaves.rrd
redis-example/total_commands_processed.rrd
redis-example/total_connections_received.rrd
redis-example/uptime_in_seconds.rrd
redis-example/used_memory.rrd

Enjoy! :)