Openfisca-core

* Implement simulation.calculate `print_trace=True` argument. Options: `max_depth` and `show_default_values`

Examples:
python
simulation = scenario.new_simulation(trace=True)
simulation.calculate('irpp', 2014, print_trace=True)
simulation.calculate('irpp', 2014, print_trace=True, max_depth=-1)
simulation.calculate('irpp', 2014, print_trace=True, max_depth=-1, show_default_values=False)

* Load extensions from pip packages

* Use DEFAULT_DECOMP_FILE attribute from reference TB system

* Explicit the error when a variable is not found

* Update numpy dependency to 1.11

* Force updating version number and CHANGELOG.md before merging on master
* Release tag and Pip version automatically