Support registering RPC service methods without an HTTP request parameter

This change allows registering service methods that conform to the standard Go library RPC method signature:
>```go
func (t *T) MethodName(argType T1, replyType *T2) error
```
http://golang.org/src/pkg/net/rpc/server.go#L23

The feature is exposed by a new `Server` function `RegisterTCPService`. The behavior of the existing function `RegisterService` is unchanged.

E.g.:
```go
s := rpc.NewServer()
s.RegisterTCPService(new(Arith), "")
```

Please let me know if you have any suggestions or concerns.