Add option for server logging

This commit adds an option to enable server logging through connect's built in [logger](http://www.senchalabs.org/connect/logger.html). 

Usage:

```js
browserSync.init(['build/**'], {
  server: {
    baseDir: 'build'
  },
  logger: 'dev' // or any other value accepted by connect.logger()
});
```

![image](https://cloud.githubusercontent.com/assets/647549/3180985/eb63621c-ec38-11e3-90b7-be4abcaae8d9.png)
:point_up: Pretty!

I couldn't quite figure out how to your tests running, so one still needs to be written if you think the change needs one.