New backend for InfluxDB metrics

This adds a new datasource for fetching metrics from InfluxDB. That project is still very new, so the API could very well change, but it's so easy to work with it's almost stupid not to do this.

/cc @eric @pauldix @auxesis because they seem generally interested in this sort of :poop:.

```
$ cat dashboards/foobar.js
var metrics = [
  { "target": "foo" },
  { "target": "bar" }
];
```
```
$ INFLUXDB_URL=http://sandbox.influxdb.org:9061/db/foobar INFLUXDB_AUTH=foo:bar foreman start
```
![screen shot 2013-11-08 at 5 43 34 pm](https://f.cloud.github.com/assets/494338/1505608/92c70efe-48de-11e3-9509-4ff167033ef2.png)