NewLoggingHandler: handler with optional logging of req/resp headers

This is a new class because we would have had to change the API of the old LoggingHandler.
Add a base Metrics trait for app metrics
Add Jetty handler/request metrics and regular logging of metrics
MetricsOptions - be able to configure logging interval, prefix, etc.
CR: Use getHeaders() to retrieve multiple header values
Oops, fix typo
Remove all the metrics stuff (moved to thirdparty-utils)
Add an option extraHandlers to inject extra Jetty Handler wrappers
CR from @znep
Add a README.
Merge pull request #3 from socrata/velvia/logging-handler-enhancements

NewLoggingHandler and extraHandlers