Add the ability to specify a url_to_path table.

This allows files to be looked up that may not be in the root path.
Clean up the test for url_to_paths functionality.
Update docs for the new url_to_paths option.
Add url_to_paths to initial example in README.md
Beef up the test for url_to_paths.

* Add a file to the repo, test/test-data/test-file.js which is used as a stable file from which to calculate the md5. This is used to make sure url_to_paths is operating as expected.
Merge pull request #27 from shane-tomlinson/url_to_path

Add the ability to specify a url_to_path table.