Fix bug of start_time_entry

When I used start_time_entry method,  I got the following output.
```Toggl::BadRequest: (400): created_with needs to be provided an a valid string```

So, I check toggl_api_docs.
https://github.com/toggl/toggl_api_docs/blob/master/chapters/time_entries.md

I found the following description.
```created_with: the name of your client app (string, required)```
To see this, created_with is required property.
But, start_time_entry method don't add this property.
So, add this property.