[Reviewer: Andy] Escape all fields, and do it properly.

Andy,

Please can you review my fix to escape values when doing replacements in local_settings.py.  This fixes #80.

I've live-tested.

Thanks,

Matt
I think you need to also do this to `\` (to prevent us trying to escape other characters) and `&` (which tells `sed` to insert the matched string into the output).