Re-implemented RSSWatcher to use asyncio.

 - safeexec() distinguishes normal functions and coroutines.

 - Now it properly checks timeouts for coroutines.  If timeouts happen,
   a TimeoutError exception will be raised and shown to users by
   sayerr().
Reduce default retry interval of RSSWatcher.

 - A value longer than botimpl's TIMEOUT is 20 seconds is meaningless.
Merge branch 'master' into name-change