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.
id rather waste one variable for the result for readability (really just segmenting branch logic and re-use variable) and make it inline with the other branching logic..  while we dont stick to the whole 80 char per line, this is just asking for trouble. yes there are pythonic ways to do things, and then there are pythonic ways of doing something but keeping it readable/maintainable to others :)