Expose use of the event loop

Expose use of the event loop

The use of the event loop where previously hidden
in internal deep places of the websocket modules.

This commits enforce the client of the library to
explicitely tell what event loop must be used.
This has 3 effects:
 - you know that the systemd event loop is used
 - you tell the event loop to use (no confusion)
 - you don't depend on afb-common.c

Change-Id: Id13d8a96f981183c299cde414d9bb0cd77fe3daa
Signed-off-by: José Bollo <jose.bollo@iot.bzh>