Improve falcon client

Summary:
Using new tool for parsing command line options. It is now possible to
start/stop/restart the daemon. It is also possible the specify several files to
be dirty.

Note: the client can be given several targets to be built, but on the server
side we currently only build all the roots by default.

Test Plan:
Try the following commands:
falcon.py --start
falcon.py --restart
falcon.py --stop
falcon.py --get-dirty-sources
falcon.py --set-dirty ../src/main.cpp ../src/depfile.cpp
falcon.py --build
falcon.py --restart --build
etc...

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D38