Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

The `WebClient` has a new exception hierarchy:

* `WebClientException` is the root of all exceptions thrown by the
`WebClient`
* `WebClientResponseException` are all exceptions associated with
specific HTTP response status codes
* `WebClientErrorException` and `WebServerErrorException` are
respectively for 4xx and 5xx HTTP status codes

`ResponseExtractor` implementations are adapted to optionally throw
exceptions if it's impossible to extract the relevant parts of the
response (e.g. extracting the response body if the response is a 404).

This commit also introduces `ResponseErrorHandler`s that take care of
the whole exception mapping infrastructure. Since
`WebClientResponseException`s provide the status, headers and response
body, we also need a dedicated mechanism to extract information from the
response body at that level.

The `BodyExtractors` are responsible for extracting that information
from the exception, given they are provided with all the information
they need; in that case, message decoders are required.

To convey all this new information downstream, the `WebClient` now wraps
the message converters and response error handler instances into a
dedicated `WebClientConfig` object.

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 the `WebClient`, which relies on several parts of our
infrastructure:
* a `ClientHttpRequestFactory` to drive the underlying client library
* a `HttpRequestBuilder` builder API to create the client request
* a `WebResponseExtractor` to extract the "low-level"
`ClientHttpResponse` into a higher level representation such as a
decoded body

The `WebResponseExtractors` helper class contains many extractor
implementations all based on the `Flux`/`Mono` composition API.

This commit adds a `DefaultHttpRequestBuilder` and its companion static
builders in `HttpRequestBuilders`. This allows to build client requests
with a friendly builder API, inspired by Spring's MockMvc API.

This commit adds the base contracts for the Web client.
The "Reactive" prefixes of the previously commited contracts has been
removed to match the server ones.

Both the `ClientHttpRequest` and the `ServerHttpResponse` extend
`ReactiveHttpOutputMessage`, which now has a `beforeCommit` method,
necessary in both client and server implementations.

`HttpRequestBuilder` will be used by the developers to create requests
with a nice builder API. `ClientHttpRequestFactory` will provide support
for many HTTP client libraries in this new client.