Allows specifying a directory to the provider configuration

This attempts to address #12 by adding a directory parameter to the provider options.  The example in the readme would mean that mongoose-attachments does any ImageMagick related transformations in `os.tmpdir()`, then mongoose-attachments-localfs would move the transformed image into `/absolute/path/to/public/images`.

I've also added unit tests with code coverage for the above.  To run the tests do:

```
npm test
```

..and open coverage/lcov-report/index.html to see the report.