Begin 'nodespaces' project; multiple namespaces.  Broken.

--HG--
rename : data/exhibits.yaml => data/installation/exhibits.yaml
Almost rendering.  Clumsily.  Still broken.
Yep, still broken.
Yes and well OK.
Unbreak rendering!
Get it rendering the right files in the right output directories.
Break things intentionally, again!  Ain't I a stinker?
I'll let you know when it's not broken anymore.
Faster, pussycat!  Refactor!  Refactor!
It renders, again.  Almost.
It's picking the wrong template because it can't tell the space.
One small change lets it render.  Links are incorrect, of course.
OK, so that's why we have our own pathname2url() I guess.
Yay!  It's mostly not broken!