Add codeless conditional rendering features

To allow straight forward conditional rendering two new features have
been added:

actionshot:ignore HTML comment

Sections of markup wrapped in the html comment

  <!-- actionshot:ignore
    IGNORE ME
  -->

will only be visible after actionshot has captured the page. This may be
useful for ignoring javascript tags which may alter the DOM in ways
which are not useful for the non-JS version of the site, facebook "like"
buttons, for example. ActionShot automatically removes these comments
before returning the page markup.

actionshot-fallback and actionshot-fallforward css classes

Adding a class of actionshot-fallback will make the element visible only
to the original ActionShot captured version of the page. Adding a class
of actionshot-fallforward will make the element visible only to the
dynamic version. Calling ActionShot.fallforward triggers this. e.g.

  <span class="actionshot-fallback">JS is for suckers anyway.</span>

  <span class="actionshot-fallforward">JS is awesome.</span>

  <script>
    ActionShot.fallforward();
  </script>

Will cause the fallback viewers to see the first message and everyone
else will see the second.