Add 'object' type support in #rpc_method.params_as

Params as 'object' are passed in `hello_json:json_object()` format
to handle_request functions.
Remove obsolete validation based on #rpc_bulk
Add custom validations for params

This change allows to define custom validations for params by
returning an anonymous function instead of [#rpc_params]
in param_info/1 callback functions.

The anonymous function accepts two arguments:
* 'list' | 'proplist' | 'object'.
* hello_json:json_object() | hello_json:json_array().
Merge pull request #2 from brb/validation_fun