Add serialization/deserialization for values. Fix #38

> The Client is now able to serialize and deserialize values with1
> msgpack for every commands. This behavior can be turned off with the
> Client.serialize flag or by passing 'serialize=False' to any
> command.
> 
> The behavior of msgpack (use_list and use_bin_type) can also be
> tweaked globally in the Client or locally in each command.

It breaks a few tests and needs some new ones, but I'm waiting for #43 to land before.