See #30

I've split the backend file into several modules, by matrix.org API
funcionality. Now each function receives the backend struct as first
argument and this struct is clonable so we can try to send between
threads. The backend.data is a Arc<Mutex>> so it's shared between calls
and keep the shared information.

 * backend/types.rs: enums and structs, BKCommand, BKResponse and Backend
 * backend/mod.rs: Backend struct methods, new, run and command_recv
 * backend/register.rs: Login related API calls
 * backend/user.rs: User related API calls
 * backend/media.rs: Media related API calls
 * backend/directory.rs: Directory related API calls
 * backend/room.rs: Room related API calls
 * backend/sync.rs: Sync related API calls