Add f-score, precision, recall to the output report

closes #6 (closed)

This introduces metrics such as f-score, precision and recall, as well as mechanism to plug-in new metrics. This becomes as easy as

1. Writing a formula for a metric as a function in metrics package
2. Calling the function in add_metrics.go and assign to an entry in report.Metrics map.

I think the mechanism is easy and flexible enough for future development, allowing any type of metric to be added with no constraints.