[Fix #119] Put IndexRowBuilder in "obvious" place

This was the only class in `lib` that both was clearly tied to a specific other class (`UsersHelper`) *and* didn't have standalone specs (because test coverage came courtesy of `UsersHelper`'s specs). Fixed.

RSpec: 664 examples, 2 failures
RuboCop: 180 files inspected, no offenses detected
Merge pull request #149 from jdickey/issue-119

[Fix #119] Put IndexRowBuilder in "obvious" place