Add "meta" property to event data and associated Reactors

This adds a relatively simple new field to all events:

> {  ... meta:  "Freeform set of key:value pairs e.g. { 'ewma': 12345 }", ... }

I came across this problem when working on a relatively simple use-case:

> Emit data when the metric is one (or more) standard deviations above 
> (or below) the EWMA of the same metric.

To make this decision you need to know the `metric`, the `ewma` and the `stdDev`. Before adding `meta` I was having the `ewma` stream simply attach tags:

``` js
  {
    // ...
    "tags": ["ewma:12345.67", "stdDev:234.5"],
    // ...
  }
```

This is all well and good, but it felt wrong to be serializing and deserializing these values later on in the stream processing pipeline. With `meta` I end up with:

``` js
  {
    // ...
    "meta": {
      "ewma": 12345.67, 
      "stdDev": 234.5
    }
    // ...
  }
```

and don't have to deserialize again.