A data type for proxy requests

We want to allow proxy requests to remote servers with the user's
credentials. But we don't want to allow arbitrary requests -- just
requests for URLs that have passed through this server before.

The Proxy class represents an URL we're willing to proxy for.
A route for proxying
addProxyFinisher() adds proxy URLs for remote items

If there's a remote API link, we add a proxy URL to the object before
serving it to the user. Anonymous or remote users don't get proxy
URLs.
Update header
Remove errant debug logging
Add proxy finisher to all activity feeds
Other media properties besides image
Upload a real file for proxy url test
Add the stream medialink property
Include version, better HTTP error for proxy
Retrieve proxy image, replies
clientAuth sets req.principal if client has webfinger or hostname
Rogue console loggage
Better check for data
More logging in proxy route
Use proxy URL if available for major activity
Use the version of oauth library that supports binary responses
Use oauth-evanp rather than oauth

I patched the oauth library to support binary payloads in responses,
and pushed my own version to NPM. This patch uses that library instead
of the default oauth.
Merge branch '0.2.x'