Improve serve command

[Morendil]

Fixes #634

We need to be able to specify host as well as port in openfisca serve.

Absent a specific host option, openfisca serve --bind 0.0.0.0:7777 will do, as the documentation hints at.

Discoverability of this was limited in two ways:

• the bare command openfisca crashed with AttributeError: 'Namespace' object has no attribute 'vars'
• the help option openfisca serve -h does not mention gunicorn's --bind

This PR fixes both shortcomings.

[fpagnoux]

the bare command openfisca crashed with AttributeError: 'Namespace' object has no attribute 'vars'

Hum, the bare openfisca command was not supposed to trigger a serve. The initial behavior was to crash and indicate that the command was missing. This still works in Python 2.7, but not in Python 3, as the behavior or ArgParse changed.

[fpagnoux]

Hum, the bare openfisca command was not supposed to trigger a serve. The initial behavior was to crash and indicate that the command was missing. This still works in Python 2.7, but not in Python 3, as the behavior or ArgParse changed.

Just pushed a small commit to adapt to Python 3:

Change requests were taken into account