* Add `install` usage pattern for using mixin within Tornado app
* Strip down mixin and add collector class to do actual metric recording
* Re-add path prefixes for metric type and hostname
* Add configuration parameters to disable type and hostname prefixes
`tornado.web.RequestHandler.__init__` calls
`tornado.web.RequestHandler.initialize` to initialize the object.
Sub-classes should be able to call our methods to tag metrics in
their `initialize` implementations. For this to work we need to
set our internal attributes *BEFORE* we call `super.__init__`.
