This is a preliminary draft, complete with hand-drawn figures. All the ideas that I mean to discuss are in place, but the manuscript as a whole has not been proofread with any care. In addition, there are the following more serious problems: (1) the proofs in the appendicies are correct, but some still need to be tightened up; (2) many references to, and discussions of, related work are still missing; (3) several of the arguments need to be set out much more clearly, particularly in Sections 5.3.2 and 6.3.3 and throughout Chapters 7 and 8, but also elsewhere.