Now that we have a CompositeContentTypeResolverBuilder with built-in
defaults, we switch to those defaults in places where a
ContentTypeResolver is used.

This is the equivalent of the existing ContentNegotiationManager +
ContentNegotiationManagerFactoryBean

Rename to MappingContentTypeResolver and replace "fileExtension" with
"key" in methods to be more general (e.g. file extension or query
param). Also switch from List to Set for the return values.

This commit adds RequestMappingInfoHandlerMapping and
RequestMappingHandlerMapping with support equivalent to that in
spring-webmvc.

A pretty complete equivalent to the same in spring-webmvc except for
CORS checks, and custom HTTP methods. Another notable difference is
that the "params" condition works on query params strictly.

A starting point for an alternative to the existing
ContentNegotiationStrategy but for use with ServerWebExchange.

Consistent initialization of HttpHeaders and the cookies map.
 - allow sub-classes to create instance
 - make unmodifiable

This commit adds AbstractHandlerMethodMapping, a starting point for
AbstractHandlerMapping, and HttpRequestPathHelper with a similar
purpose to UrlPathHelper but based with ServerWebExchange as input.

The new package is parallel to the "mvc" package under spring-webmvc
and a place to hold the various programming model styles.

- correct name of HttpHandlerHandlerAdapter to WebHandlerHandlerAdapter
- shorten SimpleHandlerResultHandler to SimpleResultHandler
- add HandlerResult constructor without Model
- update tests

This commit adds the "*/*" media type in the list of compatible media
types for the StringDecoder. This allows this decoder to decoder
payloads of any media type as long as the target type is String.

Fixes #87

 - Spring Framework 4.2.3.RELEASE -> 4.3.0.RC1
 - Netty 4.1.0.CR3 -> 4.1.0.CR6
 - Jackson 2.6.2 -> 2.7.3
 - Jetty 9.3.5.v20151012 -> 9.3.8.v20160314
 - Undertow 1.3.5.Final -> 1.3.20.Final

This commit underlines the fact that the request created by a
ClientHttpRequestFactory should be augmented with a message body using
the `ClientHttpRequest#setBody` method before it is executed.

See #82

This commit makes messageEncoders a required argument for building a
client request - those are needed to actually encode the body object as
a reactive stream to be written to the HTTP request body.

Removed raw types usage in DefaultHttpRequestBuilder.

DefaultHttpRequestBuilder now uses a UriTemplateHandler to expand URI
templates + variables into a concrete URI.

Fixes #80, fixes #85, fixes #86

This commit introduces the new `WebClientException` for wrapping
client-side exceptions.
This exception is now thrown when now message encoder can be found for
writing the request body as an HTTP request message.

Fixes #83

This commit adds generics support for WebResponseExtractors.
Types should be described using a ResolvableType, which can be
created with static methods like:

    // for the type Event<Registration>
    ResolvableType.forClassWithGenerics(Event.class, Registration.class)

Fixes #89

Some important issues have been fixed in recent SNAPSHOT versions.

See #78 and #70

Improve Jaxb2Decoder

- Introcuces XmlEventDecoder which decodes from DataBuffer to
  javax.xml.stream.events.XMLEvent. It uses the Aalto async XML API if
  available, but falls back to a blocking default if not.

- Refacors Jaxb2Decoder to use said XmlEventDecoder, and split the
  stream of events into separate substreams by using the JAXB annotation
  value, one stream for each part of the tree that can be unmarshaled to
  the given type.

- Various improvements in the JAXB code.

Add RequestPostProcessor

Fixes gh-81