Add redis-url and redis-socket parameters

Add additional Redis connection parameters URL and socket, in case the
user wants to connect to monitor a Redis server by using a Redis URL
that can have the following format:

* redis://server:port
* redis://:password@server:port/db
* unix:///path/to/redis.sock
Send Redis string INFO values as description

Send the values of the different Redis INFO metrics that are strings in
the description field of the report.
Send persistence last status in state field

Send the values of the persistence last status
metrics (rdb_last_bgsave_status and aof_last_bgrewrite_status) in the
state field of the report.
Sent DB keyspace metrics in the description field
Add redis-section parameter to indicate INFO section
Merge pull request #29 from inkel/improve-riemann-redis

Improve riemann-redis client