Add very basic keyring support.

Requires keyring: https://pypi.python.org/pypi/keyring

To set the password, run:

    keyring set pycarddav:$account $username
Merge pull request #77 from Stebalien/develop

Basic keyring support

Requires keyring: https://pypi.python.org/pypi/keyring
To set the password, run:
    keyring set pycarddav:$account $username