Add Modernizr for touch detection

* Detect touch devices
* Set a minimum height to match input height
* Use native date picker for touch devices
* For touch, update example text to match native date picker
Amend form method to fix validation error
Use CSS rather than JS for show and hide

* Remove JavaScript for Modernizr detection
* Hide native date of birth using css
* Move styles into date of birth stylesheet
Merge pull request #7 from alphagov/add-modernizr-for-dob

Add Modernizr for touch detection