Start adding tests for child benefit "Monday" cases we know about

These are test cases we know that should pass for the Monday-Sunday
period where no child benefit should be provided.
Add a custom matcher for commonly matched against child benefit value
Make sure we test against the end of the tax year
Add test cases for 2012/2013 tax year that return zero as value
Move StartingChild object out into a separate file and add tests
Return the same start date if on or before the 7th Jan 2013

This date is when the policy came into effect. Before this it doesn't apply.
The adjusted_start_date should return the *next* Monday

Child Benefit calculations post-07/01/2013 should have an adjusted
start date that begins on the next Monday after the provided start
date.
The only exception for date adjustment is 07/01/2013

This is purely because this date is when the policy came into effect.
The rest of the time, we should be finding the *next* Monday.
Fix the custom RSpec matcher

The custom matcher did not cause the tests to fail
when the assertion wasn't met.
Use the adjusted_start_date in the benefit claimed calculation
Add spec for 7th January 2013 use case
Adjust the date for the 2013 5-weeks test

Add an additional week's time to account for the
benefit period starting on the following Monday
Merge pull request #49 from alphagov/child-benefit-runs-monday-to-sunday

Adjust the initial day of the benefit period to run Monday to Sunday