Add payload to ICMP analyzer

This is a patch from Julien Sentier on the mailing list that makes ICMP payloads available at the scripting layer. Is there a reason this isn't already available? I would have committed it to fastpath except I don't know if it's not already doing this due to the potential overhead of creating a lot of strings in ICMP floods. At the very least, I suppose it could be optional (which the patch doesn't currently do).