Use File.join and factor out that version's output dir
Keep track of which versions were generated successfully and which failed

Where success is defined as an index.html existing.
Print out a summary of success/failure.
Fixes issue #4. Generate a global index page for all the versions.

It's really plain and ugly right now; it'd be nicer if it was in the rails guides template.
Also move utility functions into a module.
Add logging messages about index creation and how to open it when everything is done