Pipe stderr as well

Before:

>>> import keyring
>>> print keyring.get_password('aldskfj', 'slkdjf')
Error: aldskfj/slkdjf is not in the password store.
None

>>> import keyring
>>> print keyring.get_password('aldskfj', 'slkdjf')
None
Merge pull request #1 from tampakrap/pipe_stderr

Pipe stderr as well