Keeping PrEfIx and BaSe Proposals

Turtle Proposals

1. Keywords should all have the same case rules. @prefix, @base and a
should allow for upper-casing
2. Directives should all have optional trailing periods.
3. Turtle should include examples of both forms of PREFIX @prefix
directives.
4. Turtle serializes SHOULD output directives using the '@' notation with
trailing periods.

If there are no loud objections to these changes, will update the document
accordingly.

Example grammar change from gkellog:

[4] prefixID ::= '@'? [Pp][Rr][Ee][Ff][Ii][Xx] PNAME_NS IRIREF "."?
[5] base ::= '@'? [Bb][Aa][Ss][Ee] IRIREF "."?

Cheers,
Gavin