Use extras sub-folder for ancillary content

Created by: rei-vilo

Please place the sub-folders fuzzing, scripts, third-party and test on sub-folder extras, as per https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#extra-documentation.

The content of the extras folder is totally ignored by the IDE; you are free to put anything inside such as supporting documentation, etc.