Deprecate msfpayload and msfencode

See #4326, but it does not close it.
Add a link to PR #4333 in the message

That way, people can see why this is if they care to.

While we have a link to the blog post at http://r-7.co/msfvenom-2015 , I
fear that by the time spring of 2015 rolls around, we'll be on a
different blogging platform and the link may be busted. I'm certain
we'll still be on GitHub then, though.
Land #4333 with URL pointer, ty @wchen-r7

Long live msfvenom!

People who have a bug or complaint about msfencode or msfpayload should
be pointed at http://r-7.co/msfvenom-2015

See #4326 (and close it?)