Check passed URL for compliance with checks.json

grader.js has been updated to use restler to check the content
of the URL passed in as --url for compliance with the checks.json
file. To handle any potential ambiguity, either --file or --url
must now be passed to the program for it to function. If neither
are passed, the help screen will be displayed and the program
will exit.

The program has been slightly restructured to support the
asynchronous nature of the restler HTTP calls but otherwise works
similarly to its previous incarnation.