feat(rxStatusSvc): Wrapper for rxNotify.

This provides a wrapper around `rxNotify` in order to make the repetitive usage of `{ type: 'success' }` type options less necessary.

It only works with singular messages today; the feature provided by `rxPromiseNotifications` is not being wrapped.

Thanks to @chriscantu for the initial version of the awesome service provided for vcenter-ui.
Thanks to @parlarjb for updating it with awesome features that makes the service even more usable.
Two things:
1. Can you use `_.defaults` here instead of `_.merge`? Naming is a little clearer IMHO.
2. I don't think you need to check that options is defined. In my quick test, the following worked just fine:
```
var check = function (options) { 
    var defaultOptions = { x: 0 };
    if (options) {
        options = _.defaults(options, defaultOptions);
    } else {
        options = defaultOptions;
    }
    return options;
}
check(); // returns { x: 0 }
check({}); // returns { x: 0 }
check({ x: 1 }); // returns { x: 1 }
```