Added StatsHelper to centrally manage metrics naming
Added project specific BaseMailer to encapsulate logging/metrics
Updated existing mailers to use PqBaseMailer
Removed unused metric constants
Changed success log message to info
No need for logging success since ActionMailer already does this
Updated to use PQBaseMailer
Merge pull request #9 from ministryofjustice/mailer_monitor

Updated Mail sending to include log and metrics generation