add heterogenous policies for backends

[samoht]

Would be good to add a proof-of-concept backend which dispatch its keys on different backends type. Maybe using a key -> Irmin.S dispatch function. So that would be easier to store some parts in memory and other parts on disk.

[samoht]

Partially done in #43 : we can choose which store to store a given key/value pair (using the right functions).

We still need to be able to do the same thing at higher-level: ie. be able to dispatch depending on path only.