User delete flag

## User Delete Flag

* Added API method `setDeleteFlag` that will respond to `POST` or `PUT` HTTP methods (`POST` being used for setting the flag, `PUT` being used for resetting it to an empty string).
* Added 5 test cases to the test suite to handle majority of scenarios - need to add more for complete coverage of `setDeleteFlag` 

Open for revisions before it is ready for a merge - to be honest, it probably needs them...

@kentquirk 
Issue reference: #12 
I'm not sure if we spoke about this.  I feel like we did, but it was a while ago.  But, should we perhaps store the timestamp of when the tombstone was created and then the "sweeper" can define how long a thing should be alive before cleaning it up?