Use the ip from the most recent HTTP request when setting up mongo processes.

Addresses #151 

Changes:
- Use `request.urlparts` to find the host to which the request is sent, and use that host to set up mongo.
- Use IP addresses, rather than symbolic hostnames, when setting up mongo processes. These are also returned in `mongodb_uri` now.

I'd call this _ip since it's only used in this module. You're not returning a hostname.