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.
This is a inline "if else" statement, there are indention in this since writing it in one line it has special meaning in python. This is similar to the
 
*condition* **?** *value_if_true* **:** *value_if_false* 

conditional operators in other languages like C, only that in python it is written like this:

*value_if_true* **if** condition **else** *value_if_false*


This is the correct "pythonic way" (fastest and uses less memory) to do it in python 2.5 or higher

To use additional variables uses more memory (if nzb content is given = variable will be copied, which can be many MB) and makes things slower (way slower on resource restricted devices)