Get rid of "unable to add watch" warnings at startup

It your project or some deps of it does not have directories listed
in `ssync.app.src`, `ssync:start()` will output "unable to add watch"
warning. And much of projects does not have `c_src` and other stuff,
so there a lot of warnings at each startup, which is not good.

So there a patch to avoid unwanted behaviour and still keep code
up to date.

Directories specified in ssync.app.src is now watched only if they
exists on the start of ssync. If they created after ssync start, they
added to watch list dynamically using `watch_new_dirs` callback.

Also, some pointless comments are removed.

**P.S.**: didn't you think that it is more appropriate to add some kind of
lazy watch to `erlinotify`, because of it job is watching files and frontend
apps should not bother about such errors?