Final version, 92 pages, 7 figures, index of notations. Remark 4.9 in
  Version 1 is wrong. To correct it we modified the weights for the Sobolev
  spaces along the gradient trajectories (Figure 3). Proposition 4.8 in Version
  1 is now split into Propositions 4.8 and 4.9. This version contains full
  details for the second derivative estimates in Lemma 4.20