feat(index.html):added download links (osx,ubuntu)

All assets are configured via shared.yaml and are located elsewhere in
the web. This could lead to broken assets at some point, but I am just
risking it for know, knowing that it's easily done to have local
resources.

Closes #106
[skip ci]

etc/api/shared.yaml

@@ -18,8 +18,11 @@ make:
     - api
     - cli
 html_index:
+  download_base_url: http://dl.byronimo.de
   asset_urls:
     crates_img: https://raw.githubusercontent.com/rust-lang/crates.io/master/public/favicon.ico
+    ubuntu_img: http://megaicons.net/static/img/icons_sizes/6/140/16/ubuntu-icon.png
+    osx_img: http://hydra-media.cursecdn.com/wow.gamepedia.com/a/a2/Apple-icon-16x16.png?version=25ddd67ac3dd3b634478e3978b76cb74
 directories:
   # directory under which all generated sources should reside
   output: gen