Allow for project resources to live in a separate directory

This is an attempt to allow a Packrat project to keep its project resources (the `lib/` `src/`, `bundles/` directories) in a separate location. The primary startup resources will still live in the `packrat/` subfolder, e.g. `packrat.lock`, `packrat.opts` and so on, but these other parts can be made separate.

This feature is primarily intended to help support e.g. DropBox users, who might want to use Packrat for a project without having to sync the installed libraries / sources.

The trickiest thing to get right in such a scheme is the `bundle`/`unbundle` commands, since we are now trying to tar up resources from multiple paths. Currently, `bundle_internal` does that 'copy everything to one directory, and bundle that', with `unbundle` being able to untar and later move project resources back to the appropriate path.