Fix Jinja2 is required (#19)

Move common and Jinja2 files outside the templatetags/ directory
because all files of this directory are automatically loaded by Django
and that triggers an import of Jinja2.

It requires to update the path to `jinja2_globals` in your project
(README.md updated).

Author: stephane

README.md

@@ -125,7 +125,7 @@ The project provides examples of integration with Django and Jinja2 templates.
 
     ```python
     from jinja2.environment import Environment
-    from webstack_django_sorting.templatetags.jinja2_globals import sorting_anchor, sort_queryset
+    from webstack_django_sorting.jinja2_globals import sorting_anchor, sort_queryset
 
     class JinjaEnvironment(Environment):
         def __init__(self, **kwargs):

testproj/testproj/testapp/jinja2/env.py

@@ -1,7 +1,7 @@
 from jinja2.environment import Environment
 
 from django.template.defaultfilters import yesno
-from webstack_django_sorting.templatetags.jinja2_globals import (
+from webstack_django_sorting.jinja2_globals import (
     sorting_anchor,
     sort_queryset,
 )

webstack_django_sorting/templatetags/common.py renamed to webstack_django_sorting/common.py

@@ -2,8 +2,8 @@
 from django.http import Http404
 from django.utils.translation import gettext_lazy as _
 
-from .settings import INVALID_FIELD_RAISES_404
-from . import common
+from ..settings import INVALID_FIELD_RAISES_404
+from .. import common
 
 register = template.Library()