When building GitHub Apps, we need to generate a JWT.

I have an example implementation here using PyJWT, but I guess other library can work too.
https://github.com/Mariatta/gh_app_demo/blob/3dfa909866d5aa6f247b6801895a8efa7d0f3275/__main__.py#L10-L24

Perhaps this is a utility and convenient function that gidgethub can provide. (related to #67 )