Allow PHP object construction from JavaScript context

Hi again,

this fixes #15, i.e. the segfault when trying to use the constructor function on wrapped PHP objects.

Now if the JavaScript code tries to create further objects, PHP objects are transparently created as needed and attached to the corresponding internal fields.  If the PHP __construct function is hidden, an exception is passed back to V8.

Be careful, this work is based on my earlier pull request #17, which hasn't yet been merged.

cheers,
  stesie