Add to update_project a --watch option

[Daniel Lombraña González]

This will allow pbs to update automatically the project when a file is changed in the current directory.

Basically, it will allow any PyBossa developer to work on a PyBossa project with their favourite editor locally, submitting the changes to the PyBossa server of their choice. It will speed up development.

We'll use this package: https://pythonhosted.org/watchdog/