add id='global' to nav tag in 3 HTML files

move the nav from min45.25 to min0; change nav to #global; change nav to nav#global on min60em

deal with the menu; menu looks good on firefox responsive size, but a little weird when 500<width<600

make the bg less separated
Merge pull request #2 from cicirao/xi

Squashed 4 commits, manipulate the menu layout.