add members_only option. 

Haven't added tests for this pull request yet, waiting for  an OK from you.

The idea here is that in some cases you are really just interested in the `member` of the sorted set and you don't care about it's score or rank. In those cases the ranking function will have a negative impact on performance, while the results of it are discarded.

This case applies to your Ruby activity_feed library. In my Python port I could see the following difference by adding this option.

before:
![before](https://f.cloud.github.com/assets/52795/493165/16adb770-bb24-11e2-92e2-d28eaff8f735.png)

after:
![after](https://f.cloud.github.com/assets/52795/493166/1a470cb0-bb24-11e2-8531-9d7584b02300.png)