redis scheme: Use a list of values instead of a single value.

There might be some duplicates in the (key, value) pairs and we will need all of
those duplicates (not jsut one of them).  Therefore, we have to use a list.

In some applications, a single value for each key might be enough.  The list of
values works for those applications too.
redis scheme: Let redis be used as a place to store intermediate results.
examples: Add more info to the redis example.
Merge pull request #485 from pooya/redis

Redis