Add file upload functionality

Adds an :allow_uploads wiki option, an --allow-uploads flag to
bin/gollum, an "Upload" button with a file upload dialog, and a
handler to commit uploaded files into the repository.

:allow_uploads defaults to false, to prevent unauthenticated users
from uploading arbitrary files into the repository (albeit only in
the uploads directory).

This code is based on the patch from @l3iggs at
https://github.com/gollum/gollum/issues/694, but the handling on the
backend is completely rewritten to use the Committer infrastructure.

lib/gollum/templates/page.mustache

@@ -20,6 +20,10 @@ Mousetrap.bind(['e'], function( e ) {
     class="action-fileview">Files</a></li>
     <li class="minibutton jaws">
       <a href="#" id="minibutton-new-page">New</a></li>
+    {{#allow_uploads}}
+    <li class="minibutton jaws">
+      <a href="#" id="minibutton-upload-page">Upload</a></li>
+    {{/allow_uploads}}
     {{#editable}}
     <li class="minibutton jaws">
       <a href="#" id="minibutton-rename-page">Rename</a></li>