title: Get UTC offset for Timezone at given date via Ruby/tzinfo?
body: <p>This is probably trivial for anybody who knows the <code>tzinfo</code> API:</p>
  
  <p>Given a <code>Timezone</code> object from <code>tzinfo</code>, how can I get the UTC offset at a given point in time (given either in local time of the Timezone or UTC)?</p>