Consumer javascript implementation for comment

[sachss]

Hi everyone,

I've had a go at implementing the Consumer class in Javascript, reusing as much code as I could from the published Javascript Provider class. I'm short on time right now to bring this to the proper format for a PR, especially because I'm not versed in Coffeescript, but I'd really appreciate comments on the approach.

Also welcome any help bringing this to good enough condition for a pull request.

Implementation:

    var lti = require( 'ims-lti' );
    var consumer = new lti.Consumer( 'oauth_key', 'oauth_secret' );
    var requestBody = {
        lti_version     : 'LTI-1p0',
        lti_message_type: 'basic-lti-launch-request',
        resource_link_id: '0',
        resource_link_title: 'My title',
        context_id: '1234',
        context_title: "My Test",
        context_label: "No label",
        roles: 'Student'
    };
    var ltiUrl = 'https://lti.source.com/launch';
    consumer.validate_request( requestBody, function( err, valid ) {
        if( err || ! valid ) next( err || new Error( "Invalid LTI request" ) );
        else {
            consumer.send( { url: ltiUrl }, function( err, msg, response ) {
                res.send( response );
            }
        }
    }

consumer.js