#256 support redis zset

[Bugsource]

This PR support basic commands of Redis zset. For now, just only support ZADD, ZREM, ZCARD, ZRANGE, ZREVRANGE.

[KarelCemus]

Thank you for your contribution, very nice PR! Awesome work. Would you mind looking at my comments? They are very minor. I'll merge it then.

[KarelCemus]

Suggested change

	redis.zsetSize(key).map(_ == 0).recoverWithDefault(true)
	size.map(_ == 0)

[Bugsource]

I've tried with the compiler, it turned out that can't use map here cause the impact of recoverWithDefault . I checked out the implementation for set and hash, the code style is the same.

  def size = {
    redis.setSize(key).recoverWithDefault(0)
  }

  def isEmpty = {
    redis.setSize(key).map(_ == 0).recoverWithDefault(true)
  }

  def nonEmpty = {
    redis.setSize(key).map(_ > 0).recoverWithDefault(false)
  }

[Bugsource]

Hi @KarelCemus, your suggestion is reasonable, i have checked it out and optimize the code. Please check it again.

[Bugsource]

I've added test cases, but the coverage still report errors. It's strange that it didn't report errors when i commit code first time on PR.

[KarelCemus]

Thank you for your contribution, I am closing this PR since I don't have write access to your branch and creating #259 instead. I added multiple tests to fix the coverage and did some minor refactoring and addressed the comments here.