Allow users to redefine doc_type name in class Index

Fixes elastic#929

Author: honzakral

docs/persistence.rst

@@ -308,6 +308,10 @@ the index, its name, settings and other attributes:
   used for any write operations and an ``index`` kwarg will have to be passed
   explicitly when calling methods like ``.save()``.
 
+``doc_type``
+  name of the ``_type`` in elasticsearch. Note that you have to define this as
+  well as ``doc_type`` in ``class Meta`` in order for it to take effect.
+
 ``using``
   default connection alias to use, defaults to ``'default'``
 

elasticsearch_dsl/document.py

@@ -42,6 +42,7 @@ def construct_index(cls, opts, bases):
 
         i = Index(
             getattr(opts, 'name', '*'),
+            doc_type=getattr(opts, 'doc_type', 'doc'),
             using=getattr(opts, 'using', 'default')
         )
         i.settings(**getattr(opts, 'settings', {}))