minor: suppress "unable to add watch" warnings

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.
It is good to avoid bunch of "unable to add watch" warnings on erl
startup.

Also, some pointless comments are removed.
Merge pull request #11 from seletskiy/master

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