Split out CDMI code from core

[NicolasT]

Related to the changes described in https://github.com/scality/RestBlockDriver/blob/sysfs-redesign/Documentation/sysfs-interface.rst and the work started in #84.

The whole module should be split in 2 parts: one 'core' driver, srb, to which back-ends (e.g. srb-cdmi) can register (or loaded by srb when it's requested to use an unknown endpoint). This will provide a clear separation between the block access layer and the networking code.

Some code, e.g. HTTP handling, can remain inside srb (and shared between back-ends).