Parse json output in python client

Summary:
The python client now parses the json output in order to print only the commands
and their output. It now exits with error code 1 if the build failed.

Note: this uses ijson. (sudo pip install ijson)

Test Plan: Start a build with the python client.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D45