Add a 'remove_dead_links' method that eliminates references to Fedora Objects that can no longer be found in the system.
Replace custom load method with load_resource, add filter for eliminating dead entries from a compilation before show and show download actions.
Add a spec test for the case where a compilation accessed via #show has deleted records referenced in its rels_ext stream.  Clean up comment wording on the compilation model.
Don't show the 'download' link on a compilation page unless it has some downloadable content associated with it.
Update zip_compilation_job to check read permissions for each datastream it adds and add all readable content datastreams to the download, rather than just the canonical object.
Add a helper method in compilations controller for computing estimated download size, display this information to users on the 'show' page.
Check if core record still exists before attempting to access from the Rails job.  If it doesn't exist skip the pid.
Merge pull request #170 from nu-lts/feature/better_compilations

Feature/better compilations