added limited support for findKeys for redis

this solution only supports the usage of findKeys
currently implemented in etherpad-lite PadManager

redis does not support getting a list of available keys by applying
filters like it is needed for findKeys so we track added and delted
keys.
adjusted findKeysTest to work with results in different order
Merge pull request #37 from cebe/feature/etherpad-redis-findKey

added limited support for findKeys for redis