Delay load backends.

[moteus]

This allows install only needed backends My idea is provide several rockspect for

• websocket-core include only modules for work with messages. This module depends from struct and bit32 libraries.
• websocket-sync, websocket-ev, websocket-copas modules that depends from core and from other libraries.
• websocket this module for compatability and it just has dependecies from websocket-sync, websocket-ev, websocket-copas

Also it would be possible provide indepndent backends as separate modules e.g. websocket-lluv

[moteus]

You can check out this rockspecs

[coveralls]

Coverage increased (+0.13%) to 90.27% when pulling ffedd2691dee15eb00a3d9710bd08ae53c796d27 on moteus:master into d95081050e078066b76523ae1992d7e781deb868 on lipp:master.