Allow multiple commands to run at once
Allow multiple commands to run at once

E.g. if you have several terminals open and are running two test
commands at once.

Closes #78

mariusbutuc/spring,ypresto/spring,maclover7/spring,oggy/spring,y-yagi/spring,oggy/spring,grosser/spring,pigoz/spring,rails/spring,slobodankovacevic/spring,universsky/spring-1,smathy/spring