Improve flow in percentile method
Use .empty? across project

Replace .size > 0 with .empty?
Remove trailing whitespace from *.rb files

README.md left untouched
Revise .empty? edit to mode.rb
Improve .map block spacing
Pefer .reduce over .inject

As per https://github.com/bbatsov/ruby-style-guide#map-fine-select-reduce-size