Understand context managers correctly

Fixes #812.

jedi/evaluate/compiled/fake/builtins.pym

@@ -39,6 +39,9 @@ class file():
     def readlines(self):
         return ['']
 
+    def __enter__(self):
+        return self
+
 
 class xrange():
     # Attention: this function doesn't exist in Py3k (there it is range).

jedi/evaluate/compiled/fake/io.pym

@@ -7,3 +7,6 @@ class TextIOWrapper():
 
     def readlines(self):
         return ['']
+
+    def __enter__(self):
+        return self

jedi/evaluate/finder.py

@@ -221,7 +221,12 @@ def _name_to_types(evaluator, context, tree_name):
     elif typ == 'expr_stmt':
         types = _remove_statements(evaluator, context, node, tree_name)
     elif typ == 'with_stmt':
-        types = context.eval_node(node.get_test_node_from_name(tree_name))
+        context_managers = context.eval_node(node.get_test_node_from_name(tree_name))
+        enter_methods = unite(
+            context_manager.py__getattribute__('__enter__')
+            for context_manager in context_managers
+        )
+        types = unite(method.execute_evaluated() for method in enter_methods)
     elif typ in ('import_from', 'import_name'):
         types = imports.infer_import(context, tree_name)
     elif typ in ('funcdef', 'classdef'):