Add some comment and do some rewrite

Rewrite Records.load() to more clearly express the logic.
Change format of TODO
Todo session span accuracy issue

Session span accuracy is miss-matched between file and application
Todo describe the background of this progject in README
Add average span statistic text
Todo best achievable span
Update average span text in real time
Change statistics colors
Add max statistics text
Add statistics days text
Lower hue for statistics expect span color
Use max hue & minus sign if exceeds expect span
Merge branch 'dev'