Use show_bunting? model method in Welsh BH page.

Was missed then the English variant was updated.
Update bank holiday markup to be valid.

Previously had multiple <thead> elements per table which isn't valid.  It also causes some strange issues when printing (the browser repeats the wrong heading when a table spans 2 pages).
Tweak styling to ensure consistent column widths.
Improve print layout

Added a calendars-specific print stylesheet.
Merge pull request #43 from alphagov/fix_markup

Improve bank-holidays markup