Follow get_model changes in Django 1.7

get_model returns None if the application has not be ready yet until
Django 1.6 but it now raise AppRegistryNotReady in Django 1.7.
This changes cause exceptions in Django 1.6

Django 1.6:
https://github.com/django/django/blob/75d03c775553b9778db513213376d335d23819a8/django/db/models/loading.py#L259

Django 1.7:
https://github.com/django/django/blob/d92b08536d873c0966e8192e64d8e8bd9de79ebe/django/apps/registry.py#L186
Remove comment app which is deprecated

django.contrib.comment is deprecated from Django 1.7 and will be removed
in Django 1.8

https://docs.djangoproject.com/en/1.7/ref/contrib/comments/
Define include or exclude in ModelForm

Creating a ModelForm without either the 'fields' attribute or the 'exclude'
attribute is deprecated.
Add alternative library of django.contrib.comments
Regulate testcase of Products to fit on Django 17
Set django 1.7 to tox
Fix tox typo
Fix runtests.py
Fix typo
Support Django 1.7 migration
READMEを1.7対応した
READMEのバージョンが1.6のままだったところ修正
Merge pull request #207 from kawazrepos/support_django17

Django 1.7 対応