Webcredits spec moved from wiki to respec

A couple of people requested that I move the webcredits spec from the CG
wiki [1] to a more readable respec form.  So I thought I'd give this a try.

https://webcredits.github.io/spec/

In terms of maturity it should be considered a very early draft / work in
progress, ive only spent a few hours on it and most of that is learning how
to use respec.

It's aimed at being a very simple way to do linked data payments.

Conceptually you have a ledger with which gives each user a balance.  Then
you have a web credit which acts as a way to change the ledger.  A wallet
ties both items together.  That's it.

Feedback from the community is welcome!  If there's interest we can do an
issue tracker etc.

[1] https://www.w3.org/community/webpayments/wiki/Web_Credits