Structs to Interfaces

This adds a command "gozmqfix" that will update code when there are backwards incompatible changes in the API of the "github.com/alecthomas/gozmq" package.

The code is copied from the Go project at version 1.0.3 with its licensing intact.  Based on a little research into licensing, I've added a copyright line near the top of the LICENSE file.  I am not a lawyer.

Install with `go install github.com/alecthomas/gozmq/gozmqfix`.  Usage is essentially the same as for `go fix`.

Finally, change interfaces to structs!

This started with some global search and replace, then cleaned up as necessary to get things building and passing tests again.  Updating existing code should be as simple as:

    go install github.com/alecthomas/gozmq/gozmqfix
    gozmqfix PATH...

I've tested this on the files in imatix/zguide/examples/Go and it works fine (though a couple examples are missing the time.Duration update at the moment).