add RegisterServiceEntry function

[oliverpool]

Following #52 (which seems stale), this adds a RegisterServiceEntry function.

What's the benefit over Register?

To be able to override specific fields of the ServiceEntry (the HostName for instance, without having to provide ips).

Given there should be an extended API, code should be reused rather than duplicated.

I refactored the Register and RegisterProxy to use this new method.

Also, test coverage should be added.

I have added some basic tests, to check that the fields are properly set.

Let me know what you think!