Fixes issue #3

This changes the public API for `Listen()`, but `godoc` doesn't list any projects depending on yours, and you have the disclaimer noting that the API is subject to change. Moreover, the API change adds a returned `error` value, which is important for stability. I didn't add tests to validate fix this because I didn't really know how to mock the `net.Dial()` stuff, so please take care in reviewing these changes.

Merci beaucoup :)