This commit also fixes an issue in the HTTP client that used the
wrapper type instead of the element type. As a consequence, due
to type erasure, we now have to specify the type of the content
in DefaultHttpRequestBuilder#contentStream().

This commit removes the need for the custom Subscriber in
JsonObjectEncoder, and replaces it with higher-level Flux and Mono-based
solution.

Added DataBufferAllocator parameter to Encoder.encode, and use that instead of AbstractAllocatingEncoder base class.

-> reactor.Mono : reactor.core.publisher.Mono
-> reactor.Flux : reactor.core.publisher.Flux
-> reactor.Processors : reactor.core.publisher.Processors
-> reactor.Timers : reactor.core.timer.Timers
-> reactor.Subscribers : reactor.core.subscriber.Subscribers

Flux and Mono are used both for implementation and exposed at API
level to express 1 versus N semantic and to provide default Rx
operators:
- Flux<T> for multiple values Publisher (issue #48)
- Mono<T> for single value Publisher (issue #50)
- Mono<Void> for Publisher with no value (issue #49)

reactive.codec -> core.codec
reactive.io    -> util

These may very well not be the final locations. For now they simply
express that there are classes that belong somewhere in core, i.e.
they are lower level dependencies than web.

This commit adds support for Publisher based codecs that allows to convert
byte stream to object stream and vice & versa.

Jackson, JAXB2 and String codec implementations are provided.