Fix $.globalMessenger().hookBackboneAjax(). 

Previously it was not accepting options & potentially clobbering options that did exist.

I thought it strange that there was an existence check for Backbone.Ajax, as it is always guaranteed to exist.

I removed the check & tightened up the method somewhat so that it correctly accepts options on individual calls without clobbering options fed to hookBackboneAjax or globally set options.

This enables nice calls like this:

```javascript
this.model.save(data, {
  messenger: {
    successMessage: "Successfully Updated Model!",
    progressMessage: "Updating Model...",
    showSuccessWithoutError: true
  }
});
```