Introduce /api/annotations, to fetch multiple global annotations at once.

Example usage:

```
$ curl 'http://tsd/api/annotations?start_time=1415921915&end_time=1415925922'
[{"tsuid":"","description":"Testing Annotations","notes":"This is annon2","custom":{"key2":"value2","key1":"value1"},"startTime":1415921915,"endTime":0}]
```