Introduce handling of missing metrics

[ashleyprimo]

This is primarily a fix for #3, with a few added extras.

Changes / additions:

• Handling of absent metrics.
• Introduction of a log library; with the ability to alter log level output. (via -log.level flag)
• Fixed metricsPath; before it was not used since /metrics was hard-coded

I'd welcome some suggestion on how to make my method of "Handling of absent metrics" cleaner if there any better ideas floating around :+1:.

@BugRoger