title: Lua check if a table is an 'instance'
body: <p>Given a table, is there a way to check if it is an instance object of any class?</p>
  
  <p>Assume all class definition looks like:</p>
  
  <pre><code>Class = {}
  Class.__index = Class
  
  function Class.new()
      return setmetatable({}, Class) -- returns an instance
  end
  </code></pre>