Update everything to the latest API change

Now a store creator (S.create) takes a static configuration (a
dictionnary of typed keys), and task creator (typically defined by the
platform you are running on ie. Xen/Unix) and give you back a function
to call with a parameter (usually a debug string) to get back a store
handler.

Next problem: now we have a nice public API, the unit-tests can't
access the private parts anymore. Damn ...