basic syslog abstraction and concreate classes for syslog and file loggi...

This is a WIP, this is a little roadmad and some questions I have.

- [x] Create `server.Logger` interface and add `server.SetLogger` method
- [x] Modify all the actual call to the new format
- [ ] Network syslog
- [ ] Tests

Questions:
- Is really needed the [`alreadyFormatted`](https://github.com/apcera/gnatsd/blob/master/server/log.go#L57
) function? We can control this in the codebase
- Some [options like LogFile, FidFile](https://github.com/apcera/gnatsd/blob/master/server/opts.go#L45) are related more to the daemon than the Server class, we can move away? and keep it at the gnatsd.go?

Related to: https://github.com/apcera/gnatsd/issues/7