New "with ignored(*exceptions)" context manager

One of the best things in [this talk](pyvideo.org/video/1780/transforming-code-into-beautiful-idiomatic-pytho) is the `ignored(*exceptions)` context manager. I immediately grepped my projects and saw that Jedi can be improved by this -> 51 additions and 73 deletions :)

```
+@contextlib.contextmanager
+def ignored(*exceptions):
+    """Context manager that ignores all of the specified exceptions. This will
+    be in the standard library starting with Python 3.4."""
+    try:
+        yield
+    except exceptions:
+        pass
```

Watching the linked talk is recommended.

PS: The `ignored` context manager will be in Python 3.4's stdlib.