Multiple targets support

Not sure if it's a perfect solution, but this adds support for multiple targets at the same time. It outputs JSON encoded strings as keys.

Example view (`location` and `trackableValue`):
```js
"abtest": {
  "count": {
    "location": true,
    "trackableValue": true,
    "eventName": "trackable-custom",
    "trackableType": 'abtest-view'
  }
}
```

Example result:
```js
[{
  "key": 1414778400000,
  "value": {
    "{\"location\":\"http://127.0.0.1:8080/abtest\",\"trackableValue\":\"B\"}": 25,
    "{\"location\":\"http://127.0.0.1:8080/abtest\",\"trackableValue\":\"A\"}": 22
  }
},
{
  "key": 1414782000000,
  "value": {
    "{\"location\":\"http://0.0.0.0:8080/abtest\",\"trackableValue\":\"B\"}": 1,
    "{\"location\":\"http://0.0.0.0:8080/abtest\",\"trackableValue\":\"A\"}": 4
  }
},
{
  "key": 1414785600000,
  "value": {
    "{\"location\":\"http://0.0.0.0:8080/abtest\",\"trackableValue\":\"B\"}": 1,
    "{\"location\":\"http://0.0.0.0:8080/abtest\",\"trackableValue\":\"A\"}": 1
  }
}]
```

/cc @dominictarr