Added a way to configure Poulda behind a proxy with a different path pre...

I wanted to put Poulda behind an Apache server with mod_proxy with a path prefix.
Pyramid allows to do this using the route_prefix option.

I've added this change to make it possible trough configuration.
```
pyramid.route_prefix=/upload
```

In the configuration I've also added this to make it work with https
```
filter-with = proxy-prefix
[filter:proxy-prefix]
use = egg:PasteDeploy#prefix
scheme=https
```

Here's the proxy configuration part for Apache 
```
[...]
        <Location /upload>
                ProxyPass  http://127.0.0.1:6543/upload retry=5
                ProxyPassReverse  http://127.0.0.1:6543/upload
                allow from all
        </Location>
[...]
```