Librato composite metrics

Graphite supports functions such as `sumSeries`, `divideSeries` and `multiplySeries`. Those are very useful to compose metrics and report rates.

These patches add support to something similar when using librato, with the `compose` query param to the `/check` API call. It holds the name of a function (currently `sum`, `divide` or `multiply`) which will be used to combine multiple metrics, passed as a comma separated list via the `metric` param.

Usage:

`GET /check?metric=instances.healthy,instances.total&compose=divide&min=0.2&range=120`

The url above checks if the rate of healthy instances has been at least 20% in the last 2 minutes.

These changes shouldn't affect what is currently working.