31 pages, 6 Postscript figures; to be published in Calc. Var. Partial
  Differential Equations. Version history: Changes in v2 w.r.t v1 only concern
  metadata. V3 contains some minor revisions and additions w.r.t. v2. V4
  corrects a confusing typo in one of the formulas of the appendix. V5 is the
  definitive version that will appear in print