Asynchronous processing

[sharpner]

TODOs:

• Nice handling of url creation
• Find a clean solution to handle subtrees e.g. /jobs/queues/1 like in the spec

[sharpner]

So I kinda got this to work, but the tricky part is now the correct handling.
The actual flow of asynch operations would be:

POST /photos

Create gets called, but it does not create a photo, instead it creates a temporary handle. In the example
it creates /photos/queue-jobs/5234 that only contains a "status" attribute.
Optionally you get a Content-Location Header with the optional full resource identifier.

GET /photos/queue-jobs/5234

This call will return 303 See Other when its done. Additionally it contains a Location Header back to the actual Photo that got created. E.g. Location: https://example.com/photos/4577
No documentation as to what is if the process is not yet finished.

IMHO api2go does not yet have the functionality to implement such an behaviour. A user can probably fake it after this PR gets merged (without the optional location header) but to implement the suggestion 1:1 we would need a way to respond to Create differently than either Succeed or Error.

What do you think @wwwdata?

[sharpner]

RFC for #243

[wwwdata]

so far this looks good in my opinion

[sharpner]

so we are going to merge this for now?

[wwwdata]

It cannot hurt to merge it but we should mention something about it in the Readme as well