SP.Appcontext for hosted sharepoint apps

https://msdn.microsoft.com/en-us/library/office/dn531433.aspx#bk_ListGetItems

http://sharepoint.stackexchange.com/questions/136482/sharepoint-rest-api-using-caml-query-and-select-expand-to-get-lookup-data

http://blog.ctp.com/2014/04/28/performing-rest-operations-on-host-webs-from-sharepoint-hosted-apps/

http://sharepoint.stackexchange.com/questions/76645/sharepoint-2013-rest-possible-to-upload-accross-site-collections-with-embeded