Add timeout support for http/https requests

[dustymugs]

Title

Add timeout support for http/https requests

Motivation

Requests supports timeouts and their own docs recommend setting a timeout for production use.

Timeouts

This is especially true in cloud envs (e.g. AWS Lambda) where you are charged by the millisecond and can only be so patient for an HTTP endpoint to respond.

Work in progress

If you're still working on your PR, include "WIP" in the title.
We'll skip reviewing it for the time being.
Once you're ready to review, remove the "WIP" from the title, and ping one of the maintainers (e.g. mpenkov).

Checklist

Before you create the PR, please make sure you have:

• Picked a concise, informative and complete title
• Clearly explained the motivation behind the PR
• Linked to any existing issues that your PR will be solving
• Included tests for any new functionality
• Checked that all unit tests pass

[mpenkov]

Thank you @dustymugs ! Congrats on your first contribution to smart_open!