Implement module backend

"module_backend" - is a new module type from elliptics.
It is possible to define your own elliptics modules in separate
shared libraries, and load it on start.

Also:

* Add -H option to dnet_ioclient
    This option make possible to pass key "AS IS", without hashing.
    This done by test reasons

* Add C++ bindings for module_backend
    You can use HonestCommandHandler for straightforward elliptics
    handlers, or simplified UncomplicatedHandler. Those classes
    interfaces are yet not complete.