Merge pull request #63 from embaldridge/master

Adds Zipf distribution
Editorial work in the Results section
Remove reference to (now fixed) positive log-likelihoods
Indicate To Do items with TODO
Add introductory paragraph to Discussion
Consolidate TODOs for Discussion
Minor edits to Discussion
Merge pull request #64 from ethanwhite/more-ch1-edits

More Chapter 1 edits
Finishes modifying graphs for the Zipf distribution.
Merge remote-tracking branch 'upstream/master'