fix find semantics

jQuery::find finds nothing if no selector is given. This is the correct
behaviour.
Merge pull request #176 from davidchambers/fix-find-semantics

fix find semantics
propagate syntax errors caused by invalid selectors
fix semantics of next and prev
simplify siblings
simplify each
Merge pull request #177 from davidchambers/propagate-errors

propagate syntax errors caused by invalid selectors
remove redundant return value
Merge pull request #1 from MatthewMueller/master
Fixed merge conflict from master

Addign simplified $.siblings() by @davidchambers in d9a517b.