Backend work for implementing zip download of compilations.
Adds a require 'zip/zip' line to the RubyZip gem.  Gives us access to Zip::ZipOutputStream
Moves the download action above the resources :compilations call, so that it gets matched.
Adds the download compilation link.
Implements a job that zips the entries in a compilation and puts them in the /tmp folder.  Not quite as far as sending the file back to the user yet.
Modifies the zip job to install to a saner directory, and clean away that directory if it exists before writing a new .zip file.
Changes the download aciton to show_download, which kicks off preparing the file for DL.
Link on the show page now fires off the show_download action.
Adds a view for the show download action.
Changing isPartOf to isMemberOf - isPartOf will be used for the forthcoming new datastream objects
Finishes up compilations.  They now zip properly and can be downloaded as a zipfile.
Merge branch 'develop' into feature/compilations

Merging in the large refactor of the GenericFile object into a composite core type object called Nu-Core.
Adds an 'Add to Compilation' link to the actions on a file show page.
Adds an 'Add to Compilation' link to the actions on a file show page.
Merging to fix something.
Removes Sufia::Engine namespacing on the nu_core_files_path
Adds a modal dialog on the nu_core_files#show action that lets a user add this file to their compilation.
Adds a working query that returns a Collection's children.
all_child_files now returns an array of generic files associated with the calling collection, instead of the length of the array returned by solr.
Merge branch 'feature/compilations' into feature/solrize_child_collections

Merging in compilation changes to get the modal compilation popup working on the NuCollection show action.