second part of change to use local copies of dtd

keep local copies of dtd and use it

This commit ensures we don't rely on and external site for the docs to build.
If that site were to go away, we wouldn't be able to build docs.

Rearrange the binary protocol docs and xml2rfc tools.

git-svn-id: http://code.sixapart.com/svn/memcached/branches/binary@776 b0b603af-a30f-0410-a34e-baf09ae79d0b