Allow optional use of Nero AAC Encoder rather than FAAC on Windows and Linux

[jbillo]

Nero AAC Encoder is closed source but subjectively offers better quality audio than FAAC. Users will have to download and install the tool to a specific directory, but we can shell out to it if available to produce the final 2-channel AAC file. There is no OS X version available so this would only be a Windows/Linux option.