Extract logic into a function to make it usable as a library

[MattFisher]

I wanted to be able to call use autosub from another project without going through the command line via subprocess.call or similar.
So I created a new function generate_subtitles and put the logic from the main method there.
Functionality is unchanged, but it is now possible to do the following:

from autosub import generate_subtitles
new_subtitle_file_path = generate_subtitles(source_path, output, api_key=some_api_key)

I also updated the code to be compatible with python 2 and 3, though it does introduce six as a dependency.

[MattFisher]

Sorry there have been extra commits to the branch in my fork. Will fix.

[MattFisher]

Okay that's done.
I realised during more testing that 'bin/autosub.py' wasn't importable when autosub was installed. The packaging docs recommend using something like
entry_points={'console_scripts': ['autosub = autosub:main']} in this scenario, rather than scripts=['bin/autosub'], but it meant I had to move the code from bin/autosub.py to autosub.__init__.py. CLI funtionality is unchanged.

[agermanidis]

Thanks so much for this – I verified that everything works as expected.

In the future it might be better to return a Python list containing the subtitles when used as a library instead of the output path of the subtitles.

[MattFisher]

Yeah good idea!