Implementation of basic commands on server side and in python client

Summary:
This implements the following commands:
- Trigger a build;
- Get the list of dirty sources;
- Mark a source to be dirty;
- Get the pid of the daemon;
- Interrupt the build.

Test Plan:
Run the daemon, try to perform commands with the python client
See ./falcon.py -h for a description of the options available.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D17