This commit refactors the response body extraction logic into a separate
function: BodyExtractor. Standard populators can be found in
BodyExtractors.

This commit extracts the response body insertion logic into a separate
strategy interface: BodyPopulator. Standard populators can be found in
BodyPopulators.

This commit refactors the Router into a RoutingFunctions class, by:

  - Renaming the class :)
  - Moving all Configuration logic into a separate, top-level
  Configuration class with mutable builder.

Two changes to web.reactive.function:

 - Changed Response.stream method to allow for specific Publisher types
 to be returned in Response.

 - Router now stores HttpMessageReader|Writer retrieved from
 Configuration in the attributes as supplier, not as stream, to allow
 for multiple reads.

This commit introduces a new, functional web programming model in the
org.springframework.web.reactive.function package. The key types
are:

 - Request and Response are new Java 8-DSLs for access to the HTTP
   request and response
 - HandlerFunction represents a function to handle a request to a
   response
 - RoutingFunction maps a request to a HandlerFunction
 - FilterFunction filters a routing as defined by a RoutingFunction
 - RequestPredicate is used by Router to create RoutingFunctions
 - RequestPredicates offers common RequestPredicate instances