Simeon's editing pass: intro, typos, clean refs

I did a fairly careful read-through and suggest a few changes
1. Reword intro to avoid it being quite so much a of a repeat from the abstract
2. Clarify a few bits of text (especially following examples) where I don't think the subject is clear ("its" or "thier")
3. A little HTML tidying
4. Removed some references that weren't referred to