Add support for multiple gmond's to scrape.

To support that, this adds config file handling. The (json) conf specifies a list of endpoint to be scraped.

Beside that, it export metrics about the scraping process.

- gmond_exporter_scape_duration_seconds
- gmond_exporter_metrics_updated_count
- gmond_exporter_metrics_exported_count
Replace Delay by Interval.
Fix error handling: Return custom errors.
Use Histogram for scrape duration.
Send config via chan to avoid race conditions.
Merge pull request #3 from prometheus/feature/scrape-multiple-hosts

Add support for multiple gmond's to scrape.