Clean up user agents and usages of user agents

[danfickle]

This comprises the following tasks:

• Implement pluggable URI resolver.
• Implement pluggable cache.
• Make sure all calls for external resources go through the user agent. Specifically: URI resolver -> cache -> http stream.
• Allow vetoing of an external resource call, probably with the URI resolver.
• Allow overriding of the whole user agent.

[danfickle]

It is not practical to override the entire user agent so closing this for now.