Remove reliance on postal library. 

Removes the postal library and eliminates pubsub code.

Replaces PS pubsub system (e.g. for imageChanged) with EventEmitters.

This is a breaking change -- plugins will need to switch from code that looks like:
```javascript
_generator.subscribe("photoshop.event.imageChanged", handleImageChanged);
```

To something like:
```javascript
_generator.onPhotoshopEvent("imageChanged", handleImageChanged);
```

For this reason, I've tagged current master with "before-pubsub-removal"