Sentinel commands support

I've add support for Sentinel's commands: SENTINEL masters, SENTINEL slaves <master name>, SENTINEL get-master-addr-by-name <master name>, SENTINEL reset <pattern>. 

I'm not C/C++ programmer. So, please check my code. Especially function 'redis_sock_read_multi_multibulk_reply_assoc' in file 'library.c'.

Documentation is also updated.