[Reviewer: Matt] Updates to http and chronos

Matt, can you review these changes to http/chronos connection. 
The changes are:
- Opaque data is passed in as a string rather than a Json::Value (for compatibility with rapidJson). 
- Put requests no longer store the headers on a response, as Chronos doesn't update the timer id on put requests. 
- Tidying up of curl options between requests
- Delete requests with an empty timer ID will always be rejected from Chronos, so don't send it in this case.