Edits of abstract and S1, along with some random stuff throughout.
Created a new "Background" section that incorporates and organizes S1.1, S1.2, and some of S2.  Re-wrote beginning of S2 to give an overview of the approach.
Merge branch 'master' of https://github.com/dannyjacobs/fluxcalpaper
Minor formatting tweaks.
updated citations for things recently published
Incorporating changes from Adrian and Judd.
Answering residual notes from Aaron.