Add module backend

I just added module_backend - ability to load elliptics modules dynamically.
I need some feedback, and I have some questions.

1. Where can I put cpp sources of backend? Now they are lying in `test` directory, but this was made just as hack, and I want to fit to your layout.
2. What is the policy of testing in elliptics? I need to do the following: build so-module, load it with elliptics, and call test method. How can I do this?

Thanks!