README fixes.
Updating behavior for string based hooks

This commit addresses the fact that `define_hook 'after_hello'` was
treated differently than `define_hook :after_hello`. In the hook writer
it would attempt to register as a symbol (`_hooks[:#{name}]`) but prior
to that call it was registered as `_hooks[name]`
Merge branch 'updating-symbol-vs-string-confusion' of https://github.com/jeremyf/hooks
move jeremy's fixes into HookSet itself.
releasing 0.3.1.