Improve librato source

The current librato source retrieve all data from a metric, and then filter by source, but if you have many sources this will be very slow and inefficient.

This use [librato composite metric feature](http://support.metrics.librato.com/knowledgebase/articles/337431-composite-metrics-language-specification) to be able to do retrieve a metrics/sources in one call, so this will make librato API look like this:

```javascript
var context = cubism.context(), // a default context
    librato = context.librato("foo@gmail.com", "8585ae5c30d55ddef4");

var metrics = librato.metric("sum(series(\"hgsc_cpu_used\",  \"ardmore\"))");
```