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).

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 (and minor difference functionality details). However, shorter might be better. I'll change it if needed.

ZLEX is a read-only function that queries the lexicographical index of a Sorted Set.
TCL unit tests are included.

Some background info in this [SO thread](http://stackoverflow.com/questions/21311863/alphabetical-index-with-millions-of-rows-in-redis)

Kind regards, TW