Add Service Registration

Adds service registration with `dnssd.ServiceRegister`

* Looks like `dnssd_c.c` was using spaces instead of tabs.  I can put it back to spaces if that's better.
* You'll want to change [example/test.go#L5](https://github.com/dapplebeforedawn/go-dnssd/compare/soh335:master...dapplebeforedawn:master#diff-3f3679497dd4a05f6dd7499863388c44R5) to point to your repository.
* I'm new-ish to Golang, and know almost nothing about C.  I just copied your patterns.  Feedback/criticism is appreciated.