Implement Django command for dumping schema.json

* Implement Django `./manage.py graphql_schema`

* Required to have 'graphene.django.contrib' in INSTALLED_APPS setting

* Introduces two new optional settings GRAPHENE_SCHEMA and
  GRAPHENE_SCHEMA_OUTPUT

* GRAPHENE_SCHEMA for the Django app containing the schema

* GRAPHENE_SCHEMA_OUTPUT for the output file (defaults to schema.json)

* Use `./manage.py graphql_schema -h` for help in command line

graphene/contrib/django/management/commands/graphql_schema.py

@@ -0,0 +1,38 @@
+from django.core.management.base import BaseCommand, CommandError
+
+import importlib
+import json
+
+
+class Command(BaseCommand):
+    help = 'Dump Graphene schema JSON to file'
+    can_import_settings = True
+
+    def add_arguments(self, parser):
+        from django.conf import settings
+        parser.add_argument(
+            '--schema',
+            type=str,
+            dest='schema',
+            default=getattr(settings, 'GRAPHENE_SCHEMA', ''),
+            help='Django app containing schema to dump, e.g. myproject.core.schema')
+
+        parser.add_argument(
+            '--out',
+            type=str,
+            dest='out',
+            default=getattr(settings, 'GRAPHENE_SCHEMA_OUTPUT', 'schema.json'),
+            help='Output file (default: schema.json)')
+
+    def handle(self, *args, **options):
+        schema_module = options['schema']
+        if schema_module == '':
+            raise CommandError('Specify schema on GRAPHENE_SCHEMA setting or by using --schema')
+        i = importlib.import_module(schema_module)
+
+        schema_dict = {'data': i.schema.introspect()}
+
+        with open(options['out'], 'w') as outfile:
+            json.dump(schema_dict, outfile)
+
+        self.stdout.write(self.style.SUCCESS('Successfully dumped GraphQL schema to %s' % options['out']))