Looking for a CMS gem

Hey all!

I haven't used a CMS before, but a new project requires one. Rails seems
to have a whole whack of them and I'm not sure which ones match my
requirements. The library must:

- be fully themeable; the overall layout is going to be made from
scratch
- allow custom content for pages (i.e. a developer-made page)
- allow defining of different types of posts with different fields/data
- support image upload for at least pages and preferably posts as well
- be relatively easy to extract post content outside the CMS (e.g. for
use in sending an e-mail digest)

Preferably it should:
- Be compatible with Rails 4
- allow individual pages to be user-edited with HTML/CSS after they've
been created

Can anyone recommend one for me? Thanks!