Avoid creating recursive nodes for self-referencing objects. Fixes #46.

[victorandree]

This addresses the simplest case of a self-referencing object:

    <div vocab="http://schema.org/" about="http://example.com">
      <a href="http://example.com" property="url">Name</a>
    </div>

However, by tracking ancestors, it also addresses an issue where some child would try to draw its ancestors and thus itself, et cetera:

    <article vocab="http://schema.org/" about="http://example.com">
      <h1 property="name">Example</h1>

      <section rel="department">
        <h1>Departments</h1>
        <article about="http://example.com/department1">
          <h1 property="name">Department 1</h1>
          <span property="memberOf" resource="http://example.com"></span>
        </article>
      </section>
    </article>