Set __name__ = "__vapoursynth__"

Allows vapoursynth scripts to know if they are being run under
vapoursynth or python. Allows the same mechanic python uses to tell if
script is executed or imported as a library.

to use:

if __name__ == "__vapoursynth__":
   #Script is being run under vapoursynth
   clip.set_output()
Merge pull request #115 from splinter98/vsname

Allow scripts to know if they are running under vapoursynth