Added custom :discover ability to local Ability class, mimicking the way Hydra::Ability and Hydra::PolicyAwareAbility works.
ThumnailController#can_download? customized to require :discover permission on asset.
Tests added for direct and inherited object permissions.
Changed #render_thumbanil helper to use thumbnail_path.
Updated integration test.
closes #180 and #194 and makes other changes to default search behavior
closes #174 by adding checksum info to export set manifest
Merge branch 'develop' into issue-217

Conflicts:
	app/models/ability.rb
	app/models/export_set.rb
	spec/controllers/export_sets_controller_spec.rb