Moving the bookend code to its own class.

[gmajoulet]

Moving most of the bookend code to its own component.
The API is still far from being perfect but I believe this is one step in the good direction:

• Component has to be built using bookend.build()
• Component has to be preloaded using the asynchronous bookend.loadConfig()
• Component can retrieve the config without being built through bookend.loadConfig(false /** applyConfig */), which is needed to get the sharing options on desktop
• Component is toggled on/off through the store store.dispatch(TOGGLE_BOOKEND, boolean)