Better JSON serialization of `Unit`s

This release brings better support for json_encodeing data returned by this library and minor documentation updates.

• The Unit class now includes its values in its JSON representation (thank you @1franck!):

$unit = new Unit(42.5, "°C", "hot", "2.5");
$this->assertEquals(json_encode($unit), '{"value":42.5,"unit":"\u00b0C","description":"hot","precision":2.5}');