Proper redis-cluster MGET & MSET by being aware of hash tags

[eoghanmurray]

Ref: #177

This is a fix for the non-use of MGET & MSET when in redis-cluster mode.

We (carefully) examine the keys to determine which can be grouped together as they use the same hash tag and hence go to the same slot.

[eoghanmurray]

Just now realise that there is some similar prior work on this:
predis/predis#149
I think the hash slot checks also satisfy those conditions:

• IF the key contains a { character.
• AND IF there is a } character to the right of {
• AND IF there are one or more characters between the first occurrence of { and the first occurrence of }.

This is embodied in the following logic:

        if key.count('{') == key.count('}') == 1 and \
           -1 < key.index('{') < key.index('}') - 1: