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 
Damn. I'm trying to resurrect my thinking behind that comment and what the original train of thought was on this.

I believe when I was first thinking this through, I had thought of a user being able to set a delete flag from an account management page in blip (or, by proxy, only through Tidepool-approved channels). That way someone can't have their account deleted through a potential third-party application.

The way that the function is written may indicate otherwise though. I've slept a few times since I wrote that out, so I'm still trying to recover my thought process.