use different `now` function according to the `USE_TZ` setting

Hi, I'm using django-notifications in my project and it's a great django app. But I found that there was an exception throwed(see below) when I set `USE_TZ = False` in settings.py. I think there should be a test for USE_TZ when using django's `now` function or python's built-in `now` function. So I fixed this issue and made this pull request. Please merge this fork. Thanks.

The exception's stack trace:
```
  File "/usr/local/lib/python2.7/dist-packages/django/dispatch/dispatcher.py", line 172, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/usr/local/lib/python2.7/dist-packages/notifications/models.py", line 191, in notify_handler
    newnotify.save()
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 463, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 551, in save_base
    result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/manager.py", line 203, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 1593, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 911, in execute_sql
    for sql, params in self.as_sql():
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 872, in as_sql
    for obj in self.query.objs
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 292, in get_db_prep_save
    prepared=False)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 817, in get_db_prep_value
    return connection.ops.value_to_db_datetime(value)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/mysql/base.py", line 283, in value_to_db_datetime
    raise ValueError("MySQL backend does not support timezone-aware datetimes when USE_TZ is False.")
ValueError: MySQL backend does not support timezone-aware datetimes when USE_TZ is False.
```