Implemented the ZLEX extension

Hi Antirez,

I implemented the ZLEX extension on redis-server, described [here](https://github.com/tw-bert/redis-zlex/wiki/ZLEX-redis-server-extension).

ZLEX is a read-only function that queries the lexicographical index of a Sorted Set.
TCL unit tests are included.

Supersedes the ZRANGEBYLEX [pending request](https://github.com/antirez/redis/issues/324)
Development of ZLEX started before seeing this pending request, hence the slight name change. However, shorter might be better. I'll change it if needed.
ZLEX also has some minor differences in functionality, compared to ZRANGEBYLEX. This is due to use case requirements by my company. I tried to put as less restrictions as possible on querying the lexicographical sorted set index (while keeping it sound).

Some background info in this [SO thread](http://stackoverflow.com/questions/21311863/alphabetical-index-with-millions-of-rows-in-redis)

Kind regards, TW