Add comprehensive suite of mocks for testing

[sigmavirus24]

It would be nice, when people are using github3.py, to not have to mock out different objects in github3.py. Having a comprehensive suite of mocks that people can use while writing tests would be a really awesome feature.

It would allow users in their tests to use something like

from github3 import mocks

gh = mocks.login(username, password)

And to pass that into whatever code they're testing so if their test code looks like:

def fetch_authenticated_user(github):
    return github.me()

They can do

fetch_authenticated_user(gh)
gh.me.assert_called_once_with()

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/8580260-add-comprehensive-suite-of-mocks-for-testing?utm_campaign=plugin&utm_content=tracker%2F183477&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F183477&utm_medium=issues&utm_source=github).

[sigmavirus24]

Could also call this fixtures. We should probably also provide a session that is automagically betamaxified.

[sigmavirus24]

Duplicate of #805