Compute ratio of females to males

Also: log(base2) ratio
Create basic plot of log.ratio versus exam
Reorder plot categories by gender disparity
transform x-axis to log-base-2

include pretty-print math axis label
Map the size of the points to the total number of test-takers

Remove the total exams data by subsetting
Add plot title and y-axis label
Add vertical orange line to show 1:1 gender ratio
Merge pull request #1 from briandk/plot-gender-disparities

Plot gender disparities