Support pluggable job-dependencies from packages

[Iván Loire]

A package could expose job-dependencies that every job could use. Example:

"mypackage" folder:

- /dashboards
- /widgets
- /jobs
- /job-dependencies
     - /elasticsearch-logger/
            - elasticsearch-logger.js

so this get automatically expose to jobs:

module.exports = function(config, dependencies, job_callback) {
   var text = "Hello World!";
   job_dependencies.elasticsearchLogger.log({ message : text });
   job_callback(null, {title: config.widgetTitle, text: text});
};

Open questions: - Deal with name collisions? Namespacing?