Add support for document retention policies. Closes gh-issue #189.

mayan/apps/common/literals.py

@@ -0,0 +1,9 @@
+from __future__ import unicode_literals
+
+from django.utils.translation import ugettext_lazy as _
+
+TIME_DELTA_UNIT_CHOICES = (
+    ('days', _('Days')),
+    ('hours', _('Hours')),
+    ('minutes', _('Minutes')),
+)