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.
Fix some of zbr@'s code style requests

Pull-Request: https://github.com/reverbrain/elliptics/pull/66
Provide additional readme/documentation

Pull-Request: https://github.com/reverbrain/elliptics/pull/66
Fix exception handling in command handler
Do not override err with dlclose_error
Use "" instead of <> on zbr@'s request
Use only two headers to module_backend API on zbr@ request
Move source code to module_backend_t.c
Merge pull request #66 from itroot/module_backend_v_2

Implement module backend