Support sources for increment and timing.

For my use case, I needed to support sources for librato metrics.I could only find a global configuration option to do so. I tried to be as non intrusive as possible. If you want to supply a source to your metric use the following format:

    librato.increment "cpu_usage;web.production.11"
    librato.timing "queues.timelag;worker.staging.2", 234.67

Everything after the `;` will be regarded as the source. If no source is supplied its simply ignored. If the metric itself provides a source, than that takes precedence over the global configured one. This does what I need it for. Maybe its useful for others too.