Refactored the common set functionality in StructuralSets to enable use in the DisplaySet code
Generalised ModsStructuralSet to ModsSet for StructuralSet and DisplaySet usage. Further generalised permissions/set code and added DisplaySet and specs
Added DisplaySet Edit/Create forms, controllers and basic exhibits controller
Small changes to exhibits#index
Added display_sets_controller spec and fixed issue StructuralSet spec
Added display_set relationship to hyhull::ModelMethods. Updated resource views to accomodate the new display set selection.
Resource set management: Moved resource set management JS code into set-tree.js.  Resources can now include more than one set select tree per page
Resources now use set_management partial that includes option to set Structural and Display set.  Added description field to ETDs