Syntax notation for the spec

At the moment, the spec uses the same notation as the XML Rec to describe
the syntax.  Since we don't want to depend on the XML Rec, and the XML
Rec's notation is not standardized anywhere, this means we need to have a
substantial section of the spec that specifies this notation.

Another possibility would be to use the standard IETF syntax notation,
ABNF, which is described in its own RFC, RFC 5234.  This would have the
advantage that we could eliminate the notation section in favour of a
reference to RFC 5234. The JSON RFC, for example, uses ABNF.  However, ABNF
has a couple of limitations that would make it a bit inconvenient. Most
importantly, it doesn't have the difference operator (- in our notation),
which we rely on quite heavily. Also literal strings in ABNF are
case-insensitive; if you want a case-sensitive string, then you have to
spell out it out in hex.

Any opinions?

James