Implement http+unix dispatcher

[sethmlarson]

Lots of users for Docker, Sentry, etc use the http+unix://... URLs. Might necessitate additional backend interface if we intend this to be first-class support.

[tomchristie]

Any good reference describing this scheme?

I'd expect that this is more of a good case for us ensuring that the dispatcher interface is sufficiently capable, and sufficiently well documented. If we can allow third party packages to support themore complex use cases, without building them into the core package itself, then I think that'd be a great thing.

[tomchristie]

Let's treat this as out-of-scope at this point in time.