GitHub integrations support

[jayfk]

This PR adds support for the all new GitHub Integrations.

Since the GithubIntegration is the main entrypoint to obtain access tokens, I've created it alongside the Github main class.

I wasn't able to recycle much of the functionality in Requester because the integration is too coupled with the Github main class. Instead, I've used the wonderful httplib.

Usage

from github import GithubIntegration, Github

# obtained during integration creation
private_key = "some private key"
integration_id = 123

# obtained when a user installs the integration (webhook)
installation_id = 456

# get a token
integration = GithubIntegration(integration_id, private_key)
auth = integration.get_access_token(installation_id)

gh = GitHub(auth.token)
repo = gh.get_repo("user/whatever")

Tests

None, yet. Integrations are in early access right now. I think it's safe to add tests once the API is stable.

Added dependencies

In order to create the JSON web token, this PR adds python-jose. Another candidate is pyjwt, but that isn't installable on Google Compute Engine.

[jayfk]

This works, let's merge it in.

[webknjaz]

Hey @jayfk, in which case does InstallationAuthorization.on_behalf_of get populated with some value? I cannot find anything about it in docs...