Add support for automatic creation and refresh of Github App Installation tokens

[durist]

This PR adds support for automatically creating and refreshing Github App Installation tokens. It works by creating a separate thread for token refresh. It uses the requests, jwt and pause modules. This is useful since Github App Installation tokens have higher rate limits, but expire in an hour.

[Iamrodos]

This introduces external libraries (jwt, requests, pause), which the project has avoided so far. requirements.txt is empty, and setup.py builds install_requires from it. Since they aren't added there, they won't be installed, and because they're imported at module level, every install that upgrades to this would fail on startup, even for users not using --as-app-dynamic-token.

I have other comments, but this is the first hurdle. It's up to @josegonzalez whether starting to take on external package requirements is OK.

[josegonzalez]

I don't want to add external dependencies - part of the charm of the codebase was that we did everything with the python stdlib.

Not sure I care as much these days, but let me mull it over for a few days.