Rendering and Calculation Updates

Applied update which will omit certain values if omitZero set to true.
Before update even if set to true, days, hours and minutes would still
show when you would define hoursOnly, minsOnly, or secsOnly. Updated
seconds countdown to properly calculate seconds it set to true, before
it only took minutes lift into account. I also updated comments to make
sure monthsAndYears and weeks are set to false to properly calculate.

Updated Read me to include omitZero option