Add -H option to dnet_ioclient

This option make possible to pass key "AS IS", without hashing.
This done by test reasons
Add module_backend implementation

"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.
Add C++ bindings for module_backend

You can use HonestCommandHandler for straightforward elliptics
handlers, or simplified UncomplicatedHandler. Those classes
interfaces are yet not complete.
Merge pull request #62 from itroot/module_backend

Add module backend