Add support for multiple Docker API versions

Add support for Docker API v1.12

Fixes #106
/v1.11/version is an acceptable URL, and we get the added benefit of the server telling us if it supports that version or not.

Is there any reason to allow a user of go-dockerclient to pick a particular Docker remote API version to use? Normally I would say no (just always use the latest), but with the "true" issue mentioned below, it might be worth allowing someone to specify that they want e.g. v1.11.

I'm ok leaving the code as-is if you are (if you request a version, that's what we'll use; otherwise, use the unversioned URL). Or do you really want it so the client always sends the version?