Support for bucket initialization from input pipe.

* Class audiopia::resource class gets new static function 'from_json()'.

* Function audiopia::json::from_string() gets an optional argument to
  force converting to audiopia::json::string_type if first attempt fails.

* Add support for reading from json file (pipe) with audiopia-cli.

* Add forward constructor to audiopia::query_collection.

* Add default constructor to audiopia::filter_collection.