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.
can you rewrite this so it isn't on one long line (move the if/else inside the current indent branch)