Add `@json` special parameter for http json bodies

[jamillosantos]

Like @request, @client, @session, add a @json special parameter at the routes definition.

This new special parameter will accept bodies as JSON try to parse it and if fails calls the controller with a Json::nullValue as value.

Route file:

GET    /        controllers::index::def(@json)

Controller:

namespace controller
{
class index
{
     icarus::result def(Json::Value json) // JSON parsed
     {
         // ...
     }
}
}