Added support for the new append method from NZBGet 13+

NZBGet 13 will probably be released in the next few days. This version has a new combined append method that supports both nzb urls and content. Also the return value was changed form boolean to integer (NZBID or error number).

The old methods append and appendurl are now Deprecated, so it's more future proof to add the new method now.
So you waste many MB's (Full Season nzb for example) of memory, which are many not even available on resource restricted devices (and this can result in crashes on these devices) like I use....