title: Get primary key of Eloquent model instance
body: <p>I'm happy enough to work on a project with (not) very consistent column names in our MySQL database. We have camelCase, PascalCase, 'Id', ID and it's driving me nuts. I've decided to start using Eloquent in our application (which is NOT Laravel). I would love to have a method of receiving the value of the primary key easily without knowing the column name e.g.</p>
  
  <pre><code>// User's primary key column is called UserID
  $user = User::find(1337);
  $userId = $user-&gt;id(); // = 1337
  </code></pre>
  
  <p>Is there a Eloquent way of doing this or am I gonna have to add this method myself?</p>