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.
referring to: https://github.com/itofzo/Sick-Beard/commit/225db89b84a5451a75c9643360c4a6eb1f633464

nzb_content = nzbcontent64

is coping the entire content of an nzb in another variable.