Rescue against library load errors (#8)

This PR *should* fix #8: I don't have a Windows computer to exactly replicate the steps on.

Notify will rescue from a `LoadError` and return "Notify can't find the library specified." In the case of `earthquake`, which was mentioned in the issue, it will continue running without crashing. Since the notification part of earthquake *isn't* crucial, I think this is a good solution.

You can test this by running: 
```
> NOTIFY=foobar notify title message
     Notify can't find the library specified.
     title: message
```