Handle actually issuing REST requests, not just building them

[anaisbetts]

This PR adds the code to issue REST requests, which is surprisingly complicated. Based on the return value of the method, they could want:

• A deserialized JSON object (i.e. Task<Toaster>)
• A completion result (e.g. if it's a delete, so Task)
• The resulting data as a string (i.e. Task<string>)
• The actual response object (i.e. Task<HttpResponseMessage>)
• All of those things except for completion, for Rx too