Use M/D/YYYY for date format in sample records

Explanation of M/D/YYYY included in Readme "Assumptions".
Describe date sorting feature
Implement new feature steps
Add ASCII sorting to list of assumptions
Change features sample data

Changing dates to pose more of a sorting challenge and better
demonstrate behavior.
Pass "Sorts by Birth Date" feature

* New specs and code for this feature.
* All feature tests and unit specs pass.
Change block argument name
Condense specs using shared examples
Adapt executable
Adjust markdown
Extract private method
Upgrade Cucumber and add bundler :test group
Merge pull request #5 from amar47shah/date-sorting

Date sorting