Add support to update the document indexes from workflow state changes.

Add a new workflow field called internal_name for easier workflow
reference in document index templates.
Generalize the PropertyHelper class.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_states/classes.py

@@ -0,0 +1,13 @@
+from __future__ import unicode_literals
+
+from common.classes import PropertyHelper
+
+
+class DocumentStateHelper(PropertyHelper):
+    @staticmethod
+    @property
+    def constructor(*args, **kwargs):
+        return DocumentStateHelper(*args, **kwargs)
+
+    def get_result(self, name):
+        return self.instance.workflows.get(workflow__internal_name=name)