Add download counter on each book

* Closes: #40

templates/book.html

@@ -1,6 +1,7 @@
 {{template "header.html" .S}}
 
 {{$role := .S.Role}}
+{{$downloadCounter := .DownloadCounter}}
 {{with .Book}}
 <script>
 function delBook(){
@@ -52,6 +53,9 @@ function delBook(){
 		<a href="/download/{{.ID}}/{{.Title}}.epub" class="btn btn-large btn-inverse"><i class="icon-download-alt icon-white"></i> download</a>
 		<a href="/read/{{.ID}}" class="btn btn-large btn-warning"><i class="icon-eye-open icon-white"></i> read it!</a>
 		</div>
+		<div class="pull-right">
+			<small>Downloaded: {{$downloadCounter}} times</small>
+		</div>
 		</div>
 		{{if eq $role "admin" "moderator"}}
 		<div class="row"><p></p></div>