Add `readme` and `contents` from the Contents API

http://developer.github.com/v3/repos/contents/

I begin adding `archive-link` too, but it didn't play well with `api-call` since it doesn't return json. 

Here's my partial impl incase anyone wants to pick it up from here:

```
(defn archive-link
  "Get a URL to download a tarball or zipball archive for a repository.
     archive-format -- tarball or zipball
     git-ref        -- a valid git ref (e.g. master)"
  ([user repo archive-format git-ref options]
   (api-call :get "repos/%s/%s/%s/%s" [user repo archive-format git-ref] options)))
```