Adjust next Monday 'logic'

Any date before 7 Jan 2013 then start date = 7 Jan 2013.
Any date on or after 7 Jan 2013 the start_date is the following Monday.
This change broke the original test scenarios, new scenarios added.
Merge pull request #73 from alphagov/child_benefit_calculation_fix

Adjust child benefit calculator next Monday 'logic'.