added actions for handling sub documents

I created read_doc_set_idx and and read_random_doc_set_idx to able to read and choose subdocuments:
Example: 

rewards: {
	"5194189570f9688d4d00025c" : {
			"id" : "5194189570f9688d4d00025c",
			"definitionId" : "721101",
			"from" : 13,
			"rewardType" : 3,
			"amount" : 1,
			"storeTime" : 1368660117749
		},
		"5194189570f9688d4d00025d" : {
			"id" : "5194189570f9688d4d00025d",
			"definitionId" : "720001",
			"from" : 13,
			"rewardType" : 5,
			"amount" : 1,
			"storeTime" : 1368660117749
		}
}