Added a DecoratorAscolatore. Refactored PrefixAscoltatore to use it.

In order to introduce a JSONAscoltatore (#29) we needed a proper way of
'decorating' an ascoltatore.
The only place that deals with JSON should be JSONAscoltatore.

This commit removes from all ascoltatoris the JSON wrapping, leaving
it to JSONAscoltatori. The ascoltatori.build method automatically
adds that decorator, so it is transparent for current users of the
build method.

Closes #29.
Some fixes in the README.
Added HISTORY.md document.
Changed the description.
Revert "Changed the description."

This reverts commit ff9be268631e00242eb9ffbf1f4ee280c0b584fa.
Upgraded MQTT.js to 0.2.0.

Closes #39.
Wildcards in MemoryAscoltatore should use sub/unsub to avoid leaking.

Closes #35.
Bumped to version 0.3.0. Updated HISTORY.md.
merge differences with mcollina new changes