Allows specifying a directory to the provider configuration

[achingbrain]

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.