Mention supported on IE11+

…such as on Windows RT.
Updated JS API to latest spec

…mention IE support, and that Firefox uses older cancelFullScreen()
Added ms prefixes and updated relevent examples to use new syntax
missing "have" in prose
Added section on using CSS pseudo in IE

…also updated the spec section so that it didn’t sound awkward
following the IE section.
Merge pull request #946 from dstorey/update-fullscreen

Update “Building an amazing fullscreen mobile experience” to latest spec and add IE prefixes