Modern packaging

[keithel-qt]

It would be nice if sshtunnel was packaged in a more modern way - inside it's own sshtunnel package directory with __init__.py, and the ability to break up the package into multiple .py modules. I also believe this would be helpful with adding of mypy static type checking support.

[keithel-qt]

I have started to work on this (not knowing a lot about formal Python packaging - just learning) at:
https://github.com/keithel/sshtunnel/tree/modern_packaging

Anyone should feel free to continue where I have left off there if they wish.

(@keithel is my personal github account)