feature(graphite): Add a graphite utility

This adds a utility that allows you to output metrics and events to graphite.

Add the following options to your configuration

```javascript
{
  graphite_host: 'graphiteHost', // Hostname for graphite server
  graphite_metrics_port: 2003, // Port for metrics submission
  graphite_web_port: 80, // Port for graphite web interface(events are submitted this way)
  graphite_secure: false // Whether or not to use SSL while submitting events
}
```

It adds two functions:
* `writeMetrics(metrics, callback)`
 * metrics is an object containing metric label -> metric data
* `writeEvent(event, tags, callback)`
 * `event` is the name of the event
 * `tags` is an Array of strings
 

An example stack would look like this: 
```javascript
var exec = require('child_process').exec;

var async = require('async');

var BuildBot = require('util/buildbot').BuildBot;
var knife = require('util/knife');
var misc = require('util/misc');
var sprintf = require('util/sprintf');
var git = require('util/git');
var Graphite = require('util/graphite').Graphite;


exports.get_deployedRevision = function(args, callback) {
  git.revParse(this.config.treslek_dir, 'HEAD', function(err, stdout) {
    // trim leading and trailing whitespace
    callback(null, stdout.replace(/^\s+|\s+$/g, ''));
  });
};


exports.task_deploy = function(stack, baton, args, callback) {
  var opts = {cwd: stack.config.treslek_dir, env: process.env},
      graphite = new Graphite(stack.config.graphite_host,
                              stack.config.graphite_metrics_port,
                              stack.config.graphite_web_port,
                              stack.config.graphite_secure),
      deployStartTime = Date.now();

  async.series([
    function emitStartDeploy(callback) {
      baton.log.info('submitting start deploy event to graphite');
      graphite.writeEvent("start deploy", ['deploy', 'startDeploy', 'treslek'], callback);
    },
    function fetch(callback) {
      misc.taskSpawn(baton, args, ['git', 'fetch'], opts, callback);
    },

    function checkout(callback) {
      misc.taskSpawn(baton, args, ['git', 'checkout', args.revision], opts, callback);
    },

    function wait(callback) {
      setTimeout(callback, 45000);
    },

    function emitDeployTime(callback) {
      baton.log.info('submitting end deploy event to graphite');
      graphite.writeEvent("end deploy", ['deploy', 'endDeploy', 'treslek'], callback);
    }
  ], callback);
};

exports.targets = {
  'deploy': [
    'task_deploy',
  ]
};
```

This will result in two events being sent to graphite marking the start and end of the deploy. They can be graphed with the events function like: `drawAsInfinite(events("deploy"))`. You pass the tags of the events you'd like to graph, or use `*` to graph all events. The result is something like: 
![screen shot 2014-01-24 at 12 59 47 pm](https://f.cloud.github.com/assets/61822/1998813/7f4d3f0e-853a-11e3-9338-34fe615cdef1.png)