Generate configuration file instructions

This PR updates the infrastructure and instructions for generating the configuration files.  This can be taken as closing #87, as it adjusts things to make it easy for users to *manually* update the configuration.

I spent some time trying to get the automatic methods to work, but the underlying problem is that it needs to initialize all the `ConfigurationItem` objects, so it has to vist every package.  Thus, there's no way to avoid a circular dependence (e.g. generation depends on *all* packages, so it's impossible to generate the configuration at the root of any module, because it will hang up when it tries to visit all the other packages.)  So the next best solution is just to say that users will have to run the `update_configuration_files` command if they actually want to play with the configuration.

This could also be implemented as part of ``setup.py install``, I suppose, but this seems more straightforward to me.  (@astrofrog @mdboom @iguananaut ?)