add ST3 compatibility (fixes #15)

Hi,

Here are the modifications that I had to do to make it work with ST3.
I tested both Mel and Python on Win7 with Maya2013.

I don't know why, but besides python3 related modifications (both in ST2 and ST3), I had to double all backward slashes in `file_path`, otherwise Maya would not be able to source the file. Is that due to my Maya Version ? Or do you work on mac ? Anyway, you can remove this part.

Thanks for you work, this plugin is a great idea !