Update of CfC - Negative amounts

Hi all,

An update on the CfC and the one niggling issue that is currently holding
us up.

I realized late in the day that there was a resolution taken by the group
in January [1] where we agreed on the format for CurrencyAmount and this
was not properly reflected in the specification.

I submitted PR 101 [2] which amended the spec to reflect the consensus
position of the group through 3 distinct changes:


   1. Update the attribute names (from "value" to "amount").
   2. Update the format of the amount to exclude the "hyphen" as an allowed
   character.
   3. Update the currency identifier to allow any string (not restricted to
   3 char codes).

I asked the editors to merge this PR (despite their reservations) so that
we had a spec that reflected the consensus of the group.

Before merging I reverted change number 1. on the basis that it created an
ugly repetitive naming pattern that would require a number of larger
changes to rectify.

The only person opposed to this was Dave Longley. I have taken Dave's
comments on board and ask that he (and anyone else that feels the attribute
names should be revised) submit a PR proposing new names across the board
rather than in isolation. This is not a material difference from the
consensus of the group and in my opinion is good enough for us to go to
FPWD.

Change 2 has left the spec with no support for negative amounts. As such I
have submitted two other PRs [3] and [4] which reflect the two proposals
for negative amount support that can be applied on top of the original
format agreed upon by the group.

I recommend that we merge one of these before we issue the CfC. We can
continue the discussion on issue #119 [5] as to how the group wishes to
proceed beyond the FPWD.

There appears to be no objection to change 3 so this has been left as is.

Adrian

[1] https://github.com/w3c/webpayments/issues/57
[2] https://github.com/w3c/browser-payment-api/pulls/101
[3] https://github.com/w3c/browser-payment-api/pulls/111
[4] https://github.com/w3c/browser-payment-api/pulls/120
[5] https://github.com/w3c/browser-payment-api/issues/119