Add failing test for top ranking player
Fix for fetching player rank for top player

Previously the reply was being tested for truthiness for triggering the
callback. This meant the top player (index 0) would be treated as a
false reply, causing the callback to never be triggered.

Fixed by using the existential operator.
Merge pull request #1 from ludometrics/fix-top-rank-return

Fix top rank return
Call callback without rank when fetching rank.

Changes the `rankFor` and `rankForIn` functions to call the callback even
for an error or if the member isn't found, similar to the behaviour for
`scoreFor`/`scoreForIn`.

Added separate unit test for ensuring the callback is triggered in the
case of an error for scoreFor* and rankFor*.
Always trigger callback during conditional ranking

If a member ranking is skipped do to the condition of ranking not being
met, the callback is still triggered, albeit with 0 return to indicate
no records were added, in the spirit of ZADD.
Always trigger callback for percentile*

Changes the percentile family of functions to always call the callback
even on error.
Always execute callback for rankedInList*
Always execute callback for aroundMe*
Merge pull request #4 from ludometrics/trigger-all-the-things

Trigger callback function on all codepaths.