added few more plugin configurations entity_id, entity_count, entity_lis...

In my project I needed a way to easily set entity ID and list name to a custom name.
For example:
---------------------------
{
	"products": [
		{
			"pid": 310,
			"tnid": "309",
			"title": "product name",
			"status": "1",
			"description": "product descr"
                }
        ]
}

I think this could be done from plugin configuration without the needs to extend RestfulEntityBaseNode class.

These are the new parameters added to plugin config
'entity_id' => 'pid',
'entity_list_name' => 'products',
'entity_count' => TRUE,

This also adds a count of the returned items.

I think entity_id_key and entityIdKey are good! In core/entity API module are named entity keys