Return boolean values from valid?

Hi,

I ran across some unexpected behavior the other day when using the <code>valid?</code> method to validate notifications before sending. <code>valid?</code> returns <code>nil</code> _(which evaluates to false)_ if the notification is valid.  If the notification is invalid, <code>valid?</code> raises an exception.

This updates <code>valid?</code> to return <code>true</code> or <code>false</code> which is more generally expected behavior.

Thanks for writing this!