Skip non-RPC methods in RegisterService. Fixes #5

From [net/rpc](http://golang.org/pkg/net/rpc/#Register):

> Only methods that satisfy these criteria will be made available for remote access; other methods will be ignored:
>
> - the method is exported.
> - the method has two arguments, both exported (or builtin) types.
> - the method's second argument is a pointer.
> - the method has return type error.

All this PR does is add support for registering types that have exported methods not intended for RPC use, according to the above criteria.