Update src2bin.py
Update src2bin.py

After downloaded and unzipped leeno-master.zip, there wasn't any "bin" directory, thus the src2bin.py failed with the error:
IOError: [Errno 2] No such file or directory: 'bin/Ultimus.oxt'
Check if the directory "bin" exists, if not, creates it.

Tested locally, the file "Ultimus.oxt" is correctly created inside the "bin" dir.
giuserpe/leeno,giuserpe/leeno