Refactored metric_handler

Split out into simpler classes.
Add tests
Merge pull request #8 from malcyL/refactorings-squashed

Added tests and made various refactorings
Corrected links in README.md
Added metrics to record logged in users over time.
Added running instructions to README
Merge pull request #11 from malcyL/msl-readme-instructions

Added running instructions to README
Merge pull request #9 from malcyL/msl-add-user-over-time-metrics

Added metrics to record logged in users over time.