Change wording of install instructions

'Install It' could be a little vague when the posts's content included other
install instructions. This clarifies that the instructions we provide are for
using apm.
Added theme count to the site's tagline

Closes #9
Change the way we display tags

Formatting them like 'a, b and c' felt so weird to me.
Merge pull request #29 from atom-themes/design-tweaks

Design tweaks