flatten sections nav

the old sections nav was a cute little drop down that appeared when you
hovered over it, but it wasn't keyboard accessible at all. after trying
a few tricks I couldn't really get it to work, so I've decided to just
flatten it. this is better anyway because it makes the different
sections easily viewable.

resolves issue #7
update :hover styles on :focus as well

for the sake of keyboard navigation, all of the attention I pay to links
on hover need to also be true for focus

resolves issue #8
wrap main content in a main element with skip link

my header section isn't painfully long, but that's still no excuse not
to have a skip link

resolves issue #6