merge thumbnail worker

Gemfile

app/builders/learning_object/draft_builder.rb

app/controllers/chunks_controller.rb

app/controllers/learning_objects_controller.rb

app/models/learning_object.rb

app/services/learning_object_publisher.rb

app/services/thumbnail_service.rb

app/views/learning_objects/_file_upload_form.html.erb

app/views/learning_objects/_form.html.erb

app/workers/dspace_upload_worker.rb

app/workers/thumbnail_generate_worker.rb

db/migrate/20160219132305_remove_string_thumbnail.rb

db/migrate/20160219132533_add_paperclip_thumbnail.rb

lib/thumbnail/generatable_strategy.rb

lib/thumbnail/strategies/image_thumbnail_generator.rb

lib/thumbnail/strategies/pdf_thumbnail_generator.rb

lib/thumbnail/strategies/url_thumbnail_generator.rb

lib/thumbnail/strategies/video_thumbnail_generator.rb