some semantic markup changes to the Customise dialog

made dialog container a section; added ARIA attrs; used <header> and
<footer> for dialog header and footer; moved intro paragraph into
<header> so is always visible; replaced sections and h3s with fieldsets
and legends; made buttons into <buttons>; tweaked CSS accordingly; note
that some stylings not exactly as before…feel free to tweak further as
desired.
Merge pull request #9 from jasonkiss/master

some semantic markup changes to the Customise dialog