Add support for multiple Docker API versions

Add support for Docker API v1.12

Fixes #106
Also, I remember why I put in the check for both requestedApiVersion and expectedApiVersion - it's so the initial call to retrieve the version goes to /version instead of /v1.11/version (the requestedApiVersion). But doing /v1.11/version should be fine, and the Docker server will even respond with an error if the client is requesting a version the server doesn't support.