[Reviewer: Alex] Add NullSASLogger to disable SAS logging from the PingHandler

Alex,

Please can you review this fix to add a NullSASLogger to disable SAS logging from PingHandler?  This was raised as part of Metaswitch/ralf#88.

I've updated UT (as part of homestead) but not live-tested yet.

Matt
Now that `SasLogger ` is more of an interface I wonder if these utility methods should move to `DefaultSasLogger` and the other loggers should subclass off that? I don' care massively either way though.