Infer ChoiceField type from model field when in ModelSerializer

Fixes part of issue #69

testproj/articles/migrations/0002_article_article_type.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.0.1 on 2018-02-26 18:32
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('articles', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='article',
+            name='article_type',
+            field=models.PositiveSmallIntegerField(choices=[(1, 'first'), (2, 'second'), (3, 'third'), (7, 'seven'), (8, 'eight')], help_text='IntegerField declared on model with choices=(...) and exposed via ModelSerializer', null=True),
+        ),
+    ]