HTML5 compatibility

Hi Lokesh,

the current version of lightbox2 using "rel" attribute is not valid in HTML5 context. [1]

I created a version which is valid for HTML5, but not for older standards. The only possibility I see to be compatible with HTML5 and older standards would be to make the attribute configurable, which seems to be too complex to handle for a user, which just wants to include the lightbox script.

Best regards
Adrian

[1] http://validator.w3.org/check?uri=http%3A%2F%2Flokeshdhakar.com%2Fprojects%2Flightbox2%2F&charset=%28detect+automatically%29&doctype=Inline&group=0