Review Changes

I have added the changes as per the review https://github.com/flynn/go-discover/pull/2/files#r6051867

Got confused on using the vanila version of Accept in rpcplus and also in pickMostPublicIPs(). 
Have done the other changes as per the comments

Also added client_test.go to validate the server which acts like the discoverd package with test cases to validate. 
Wanted to start etcd also with it, but that would have wanted a lot of tweaks in the design. (So currently the etcd server has to be started manually.)