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
dnet_schedule_io() should wakeup only one thread when new packet has been queued
Changed write error report logic

Only return wait error (usually -ETIMEOUT/-110) if wait internal
status (assigned in write completion handler) was set to something
except -ENXIO.

-ENXIO is set by default and means no transactions were sent
v2.23.6.0
Removed unused module backend example
Use only two headers to module_backend API on zbr@ request
Move source code to module_backend_t.c
Moved to delete declaration of non-copyable objects
Introduced bind_method analogue of std::bind
Secondary indexes implementation moved from functors to object methods
Merge pull request #69 from euroelessar/master

Implementation changes due to bind_method
Made changes to preserve ABI and API in future
Merge pull request #66 from itroot/module_backend_v_2

Implement module backend
Merge pull request #70 from euroelessar/master

Made changes to preserve ABI and API in future
Comments cleanup