Properly activate shoulda in minitest

the default integration in this version of shoulda doesnt
inject itself into minitest test classes
Fix date calculation bugs

The friendly_time_diff method was doing its calculations using DateTime
values rather than dates. The DateTimes sometimes have different
timezone values (depending on how they were generated), or may be a mix
of timezone aware and timezone-unaware date classe.

I've re-implemented the friendly_time_diff method in a way which uses
Dates for the calculations. Dates are not sensitive to timezone issues.
The friendly_time_diff method actually only calculates differences at a
date-granularity so this works better.
Fix intermittent errors due to time zone issues

irb(main):001:0> Time.parse('2012-04-06 14:53:54')
=> 2012-04-06 14:53:54 +0100

which does not equal 20120406T145354Z

Use Time.zone.parse and an explicit time zone to avoid this.
Merge branch 'fixing-date-bugs' into state_pension_calculator_fixes