condor.log()

I exposed the `condor` object globally so you can log a `trackable-custom` event like this:
```
condor.log(trackableType, trackableValue)
```

Is making it global the right thing to do?
Is it ok that the entire condor object is global?
Or should I expose a different object that has only the `.log()` method?

For example:

![image](https://cloud.githubusercontent.com/assets/242760/4411503/6e65076a-44ef-11e4-8b7e-f4137598e091.png)

See #25 


Hey, let's try and get this as a part of condor itself, not in `xhr.js`. The `xhr.js` is mostly a convenience class , and if someone wants to use some other transport method that should be fine. We for example have a client for `engine.io` also and it should have the same features as `xhr.js`