Add BMesh and WM symmetrize operators

* The symmetrize operation makes the input mesh elements symmetrical,
  but unlike mirroring it only copies in one direction. The edges and
  faces that cross the plane of symmetry are split as needed to
  enforce symmetry.

* The symmetrize operator can be controlled with the "direction"
  property, which combines the choices of symmetry plane and
  positive-negative/negative-positive. The enum for this is
  BMO_SymmDirection.

* Added menu items in the top-level Mesh menu and the WKEY specials
  menu.

* Documentation:
  http://wiki.blender.org/index.php/User:Nicholasbishop/Symmetrize

* Reviewed by Brecht:
  https://codereview.appspot.com/6618059

release/scripts/startup/bl_ui/space_view3d.py

@@ -1668,7 +1668,7 @@ class VIEW3D_MT_edit_mesh(Menu):
         layout.menu("VIEW3D_MT_uv_map", text="UV Unwrap...")
 
         layout.separator()
-
+        layout.operator("mesh.symmetrize")
         layout.operator("view3d.edit_mesh_extrude_move_normal", text="Extrude Region")
         layout.operator("view3d.edit_mesh_extrude_individual_move", text="Extrude Individual")
         layout.operator("mesh.duplicate_move")
@@ -1719,6 +1719,7 @@ class VIEW3D_MT_edit_mesh_specials(Menu):
         layout.operator("mesh.shape_propagate_to_all")
         layout.operator("mesh.select_vertex_path")
         layout.operator("mesh.sort_elements")
+        layout.operator("mesh.symmetrize")
 
 
 class VIEW3D_MT_edit_mesh_select_mode(Menu):