Stable schema locations

[Cantor, Scott]

The follow-on to the earlier thread about the schema lookup is where we
want to point remote references to the schemas in the documents and
imports. There seem to be two obvious options:

- directly into svn
- putting them up on the web site

The latter would make for simpler URLs, but there would be extra work for
releases. The former would also potentially create some problems if we
moved schemas around in the source tree for some reason, or if we removed
some in future releases.

I don't like the extra work, but I kind of think we need an explicit place
to manage them for online access.

Note that what's particularly ugly about the way Spring does all this is
that whatever URLs we use for them, that has to actually end up in the
spring.schemas files to remap them into classpath resources. So the URLs
are really a kind of API we have to commit to, a very dumb web service if
you want to think of it that way.

Or we can not do this, and work on overriding the broken Spring code with
explicit namespace -> local resource mappings, but that's not a small
amount of work either.

-- Scott