Support Unix sockets via Apache HTTP client

Use the Apache HTTP client, which gives us lower-level control of the HTTP
connection. Plug in a SocketFactory which lets the Apache client speak to
Unix sockets.

NOTE: On Linux, tests will now run against unix:///var/run/docker.sock by
default if no DOCKER_HOST is specified.
Is there a reason the HTTP Client version 4.1 was used instead of a later version? When trying to use it in my Dropwizard project, it conflicts with the Dropwizard 0.7.1 Client library, which uses version 4.3.2 of httpclient. I tried 4.3.2 locally and it builds/packages just fine (though I haven't tried the socket connection yet).