Improve 3D Cursor snapping consistency and control

In Edit mode, 3D cursor snapping has some consistency issues and some (minor) lack of control. Here's what I propose :

• In the Pivot Point dropdown, add a choice between world-space and object-space Bounding Box
• then when snapping * Cursor to Selected *, add Tool Shelf dropdown options :
  • Median
  • Bounds (World)
  • Bounds (Object)

Here's why : developer.blender.org/T52355