Support custom decision graphs

[ordnungswidrig]

Extensions of the http spec, like webdav, introduce additional status codes and need support for custom decision graphs. Today, the decision graph is defined as a hardcoded set of functions calling each other in a well defined way. Injection of custom paths is very hard and error prone.

I suppose to change the decision graph execution model to support multiple graphs not defined as chain of functions in a namespace but as data. This can be a map:

{:service-available? (decision :service-available? :known-method? :handle-service-not-available?)}

where decisionexpands to a function that looks up the callback from the resource implementation and branches to known-method? or handle-service-not-available?

Liberator will provide default decision graphs for RFC2616 (what we have in 1.0) and RFC4918 for Webdav, e.g.

We should look into https://github.com/Prismatic/plumbing