Improvements to MetricsFilter

* Added optional metering of HTTP status levels ("groups")
* Added ability to configure which specific status codes should be metered
Improvements to MetricsFilter

* Added optional metering of HTTP status levels ("groups")
* Added ability to configure which specific status codes should be metered
Merge branch 'status-levels' of github.com:ziroby/metrics-play into status-levels
Code review cleanup
Much code cleanup and reduction
Removed some val types
Cleaned up a couple more kludgy things
Code review fixes
Switched from .map() to .zip()
Switched to using 'zip' instead of 'map'
README cleanup & used an unused function
Changed a val back to a def
Merge pull request #1 from ziroby/status-levels

Improvements to MetricsFilter