Move add_to_class functions to their own module

* The new module is called methods.py and found on each app.
* Add keyword arguments to add_to_class instances.
* Remove catch all exception handling for the check in and
  check out views.
* Improve checkouts tests code reducing redundant code.

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

mayan/apps/checkouts/methods.py

@@ -0,0 +1,34 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+
+def method_check_in(self, user=None):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+
+    return DocumentCheckout.objects.check_in_document(
+        document=self, user=user
+    )
+
+
+def method_get_check_out_info(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.get_check_out_info(document=self)
+
+
+def method_get_check_out_state(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.get_check_out_state(document=self)
+
+
+def method_is_checked_out(self):
+    DocumentCheckout = apps.get_model(
+        app_label='checkouts', model_name='DocumentCheckout'
+    )
+    return DocumentCheckout.objects.is_checked_out(document=self)