Use new api based on OAuth of Github Apps credentials

[SimonLab]

When moving dwylbot to a Github App (see #51), Github provides new credentials for OAuth. This new credentials don't allow us to access as much api endpoints as before:

Here we can't get the list of the public repos of the logged in user anymore. Instead of getting the list of public repos I suggest to follow the userflow described here.
So the first step after login is to display the list of all installations of dwylbot available to the user by using the following endpoint: https://developer.github.com/v3/apps/#list-installations-for-user.

• Create a simple http client for Github app that allow us to send api request to this new endpoint (Tentacat doesn't implement at this stage the api for Github Apps, linked to Replace Tentacat by HTTPoison #25 )
• Could we use a dumb Github App to run our tests or should we create a mock?
• Display the json response on the website

[ghost]

This works