Updated engine to v6.7.0

Hi Alex.

I can't say how happy I was to work on your setup. You dug your finger right onto areas I didn't really explore and benchmark yet :-)

So I just shipped a new GRMustache version which improves the native performance.

It is still lagging behind JS, though. `[NSObject valueForKey:]` is slow. So is `[NSMutableString appendString:]`. Plus GRMustache is designed for being hooked in.

I'll stop here until I feel the optimization fever again :-)

Thanks for your initial tweet, and your really great contribution.
Gwendal