Add authentication credential handling.

I have a Jenkins install that has security enabled. When I ran the `grunt` tasks, I would get errors (some like):

    Running "jenkins-backup-jobs" task
    Fatal error: Unexpected token <

I printed out what was going on and it was an HTML authentication error.

So, I added some functionality where you can specify the login credentials for a Jenkins admin user and it appears to work for me now.

I now do something like this in my local `Gruntfile.js`:

    module.exports = function(grunt) {
      var credentials = grunt.file.readJSON('credentials.json');
      grunt.initConfig({
        jenkins: {
          username: credentials['username'],
          password: credentials['password'],
          ...

where the local credentials.json (git ignored) contains:

    {
      "username": "<my username>",
      "password": "<my password>"
    }

I'm not sure this is the absolute best way to add this functionality, but it seems to get me further along. I needed to bump the `request` dependency to get the authentication handling section.

I tested by:

 * listing plugins and jobs with no credentials when security is turned on (still fails)
 * listing plugins and jobs with the right credentials when security is turned on
 * listing plugins and jobs with the right credentials after I turned off security
 * listing plugins and jobs with no credentials after I turned off security

I can add some instructions to the README. Would appreciate any feedback.