Add a demeterr-specific version tag.

The supported mechanism to do this is via "./configure", as in
"./configure --tag de-20131114".

I chose to do it in the source code instead, so that
- if someone else builds this, they can't forget to use the tag
  (they can forget to change it, but that's less bad)
- if I switch branches and rebuild, I don't accidentally reuse
  the tag
Basically, it's nicer having the fact that we use a special tag
visible to the source control system.