fix signature separator to be dash-dash-space instead of just dash-dash...

fix signature separator to be dash-dash-space instead of just dash-dash (Closes: #133)

debexpo/base/templates/email-base.html

 {% autoescape off %}{% block content %}{% endblock %}
---
+-- 
 {{ settings.SITE_NAME }}{% endautoescape %}

debexpo/base/templates/sponsor-rfs.html

@@ -87,7 +87,7 @@ Subject: RFS: {{ upload.package.name }}/{{ upload.version }} -- {{ upload.packag
 Subject: RFS: hello/3.1-4 [put in ITP, ITA, RC, NMU if applicable] -- friendly greeter{% endif %}
 
 {% include "rfs.html" %}
---{% if upload %}
+-- {% if upload %}
   {{ upload.uploader.name }}
 {% else %}
   J. Maintainer