Checks for missing arguments when calling TopicService.create()

## Summary

I added some argument checks because I noticed that you were not passing arguments when the "New Topic" button is clicked. I also added the functionality to have empty params and still use a callback if you preferred. This is a standard pattern in the NodeJs circles.

## Code Changes Summary

Defaults undefined params argument to {}
Defaults cb to params and params to {} if params is a function
Good find. In the future, you could do something like this:

```coffeescript
create: (params = {}, cb) ->
  # . . .
```

Love that Coffeescript, yo.