Fix rounding errors

Hi, Hillel said I should file a bug report, so here it is. According to there's a rounding issue happening, which makes tax calculation different between having tax calculated on a per-item basis as compared to calculating tax on the net sum (see screenshots in above mentioned forum post for clarification). As Hillel stated, Invoice Ninja is using something called "Line-level rounding", which works this way: • Round the tax amount to two decimal places for each tax line • Discard the rounding difference • Add the tax amounts of each line together to get the total tax amount • Occasionally calculate the sum of each line as a penny less or more than with document rounding This is producing a difference in the total numbers, so there should be "Document-Level rounding" used instead: • Calculates the tax amount for each line (without rounding), adds those amounts, and then rounds the total to two decimal places •…