SWFObject custom attributes

As explained in #11, I needed to add a custom class to clippy objects.

It was possible by sending custom attributes to the SWFObject.

I've just added a new parameter to the `clippy` function to allow passing custom SWFObject attributes.

README is updated with a simple example.

Note that I changed `opts` merging into deep because `swfobject_attributes` has to be a hash.

Regards,

Gauthier.