Add statsd counters

Fixes #5

Defines the following hierarchy of counters:

    certification
      |-failure
      | |- no_proof
      | `- signing_error
      `-success

    authentication
      |-forwarding
      | |-failure
      | `-success
      `-openid
        |-failure
        | |-cancelled
        | |-bad_request
        | `-mismatch
        `-success