Improve date pattern

[Following discussion on the Hackpad about dates](https://designpatterns.hackpad.com/Dates-vpx6XlVjIbE) and a conversation with @matharden about user research from Visit someone in prison, keep three inputs for date. 

Use the pattern and number attributes to trigger a numeric keyboard and hide the “stepper” using
css.

This removes the need for Modernizr, so this script is deleted.

Also reformat the _form_date.scss file to [meet the GOV.UK style guide
recommendations for Sass nesting](
https://github.com/alphagov/styleguides/blob/master/css.md).

Screenshots for smaller screens and larger screens.

![ios simulator screen shot 13 nov 2014 10 40 08](https://cloud.githubusercontent.com/assets/417754/5028066/9528a2a2-6b2e-11e4-8f00-fe75e3dc4c3e.png)

![dob-desktop](https://cloud.githubusercontent.com/assets/417754/5028068/9af870e0-6b2e-11e4-8d80-0094ac824860.png)


Do we need the `pattern` attribute? It implies use of HTML5 forms validation which, I'm guessing, would need some more guidance regards use of messaging and other aspects of the native design.

I'd be interested to know if there was any experience of using it on services as I've none myself and couldn't see any on the hackpad.