Add optional parameters for Django fields.
Add optional parameters for Django fields.

Reviewed in: http://codereview.appspot.com/4436077/

googleapis/oauth2client,jonparrott/oauth2client,clancychilds/oauth2client,clancychilds/oauth2client,googleapis/oauth2client,googleapis/google-api-python-client,google/oauth2client,jonparrott/oauth2client,google/oauth2client,googleapis/google-api-python-client