First version of ZeroFill gem

This gem replaces the ZeroFill class in mobi/mobi. It will be used in a few of our repos - there is an incarnation of this code in mobi/signage, and it will beef up mobi/mobi/lib/reporting - any presenters which group by a date (or possibly time) dimension.

Once this PR is approved, I'll publish this gem on rubygems, then add it to mobi/mobi's gemfile and remove the ZeroFill class there.

Read the readme for an explanation of functionality.

I successfully dropped this class into mobi/mobi and it worked as expected.

+1 @akdarrah @t27duck @phddoom 
(low priority!)