Fix layout in JSHint preference page

This page's layout was accidently modified when switching to the new
layout util in commit 093acc57
Make JSHint errors optional

It turned out that there are cases where JSHint errors are too strict
and should not be treated as errors in Eclipse.

Make JSHint errors optional. They can be enabled in the JSHint
preference page. Unless enabled, all JSHint errors are treated as
warnings.

Fix #71