Allow Supplying Custom Python (including Python3) Runtimes

[Miserlou]

Well, Python3 support didn't happen at Re:Invent. Bummer.

In the interim, I suggest we add the ability to provide custom Python runtimes into the handler, including Python3. Obviously, this would be an experimental/research project at the beginning, as there will be a performance hit. The benefits would be quite huge for obvious reasons, including the ability to use the hug framework.

Instructions for running Python 3 on Lambda have already been discovered:
http://www.cloudtrek.com.au/blog/running-python-3-on-aws-lambda/

Our issues would simply be adding this support into our handler and seeing what breaks, what works, and what the performance hit would be. Can we cache the initialization time the same way we cache our S3-environment variables, for instance?

Even if there is a performance hit, this support may even be worth including for non-blocking (event-driven) operations that require Python3 support.

Suggested settings syntax:

{
"runtime": "python3.5"
}

Related tickets:
#510
#499
#6

[Miserlou]

Deprecating in favor of new ticket: #793