Added ignored context manager with usage examples

I think, this
```
with ignore(Exception1, Exception2):
    # some code
```
is way more readable than
```
try:
    # some code
except (Exception1, Exception2):
    pass
```
Especially when the nested block is huge.

I've also included two examples.

For more information you can read http://bugs.python.org/issue15806 this discussion.