feat(index.html): added back-link to crates.io

* url is created per-API and features a nice crates image coming
  from githubusercontent.

Closes #105
[skip ci]

src/mako/index.html.mako

@@ -1,7 +1,7 @@
 <%
     import os
     import yaml
-    from util import (gen_crate_dir, api_index)
+    from util import (gen_crate_dir, api_index, crates_io_url)
 
     title = 'Google Service Documentation for Rust'
 
@@ -68,6 +68,9 @@ DO NOT EDIT !
         % for api_name in type_names:
             <% ad = tc[api_name] %>
             <a class="mod" href="${api_index(DOC_ROOT, an, v, ad.make)}" title="${ad.make.id.upper()} docs for the ${an} ${v}">${ad.make.id.upper()}</a>
+            % if api_name == 'api':
+            <a href="${crates_io_url(an, v)}"><img src="${html_index.asset_urls.crates_img}" title="This API on crates.io" height="16" width="16"/></a>
+            % endif
             % if not loop.last:
 , 
             % endif

src/mako/lib/util.py

@@ -832,6 +832,9 @@ def to_extern_crate_name(crate_name):
 def gen_crate_dir(name, version, ti):
     return to_extern_crate_name(library_to_crate_name(library_name(name, version), ti.target_suffix))
 
+def crates_io_url(name, version):
+    return "https://crates.io/crates/%s" % library_to_crate_name(library_name(name, version))
+
 def api_index(DOC_ROOT, name, version, ti, check_exists=True):
     crate_dir = gen_crate_dir(name, version, ti)
     if ti.documentation_engine == 'rustdoc':