Refactor metrics into its own package

This should reinforce the difference between Metrics and Checks (or in
Tessen terms, Collectors and Inquisitors respectively). All
metrics-related operations become tessen/metrics. Then checks will
live under tessen/checks.
Implement the Graphite formatter

- Implemented the Graphite metrics.Formatter.
- Updated the uptime_collector to use the new formatter and refactored
  module.