Added support to work with redis sentinel

- Added support to connect to a list of sentinels with -S parameter and to a specific service given with -s parameter. 
- Shows in the default output the ip and port of the current master server and the total number and reachable sentinels. 
- Added the checkin 'sentinels_available' which contains the number of sentinels reachable. If some of them are down the plugin returns a WARNING and shows the number of sentinels down. In case that all of them are down returns a CRIT. 
- Check parameters to ensure you use "-H host" or "-S sentinel1:port,sentinel2:port" and not both at the same time.
- Updated the version of the scipt, help output and comments with the new features.
NOTE: In case of use the script to connect with servers via sentinel is necessary to use at least the library version Redis-1.972 which comes with the Sentinel interface.