Move the Flycheck menu under Tools.

Minor modes are not supposed to define top-level
menus in Emacs. The proper thing for them is generally to
add a menu item under Tools. This particular commit
positions the Flycheck menu item just above "Spell Checking"
and renames it to "Syntax Checking"
Merge pull request #59 from bbatsov/fix-menu

Move the Flycheck menu under Tools.