Force verbose_name to be a unicode object

In some cases, <queryset_instance>.model._meta.fields.verbose_name is
a unicode object as expected, like:
'name' or 'Name of User'

but other in cases, it is a python object representation, like:
<django.utils.functional.__proxy__ object at 0xb2a82bac>

It is unclear why this is happening, but it turned up when trying to
export just the column headers of an empty queryset. By calling unicode,
it is forced to be the desired unicode object.
Merge pull request #31 from steventlamb/topic/fix_python_repr_in_empty_queryset

Force verbose_name to be a unicode object
Use django custom six module

Try to use django's six instead of cutting out six entirely, as proposed by:
https://github.com/azavea/django-queryset-csv/pull/30
https://github.com/steventlamb/django-queryset-csv/commit/ab1bd3c0cb01148518887760e4fc290c622b0518
Merge pull request #32 from steventlamb/topic/use_django_six

Use django custom six module