using Python Keyring Lib to store password

Owncloud client actually uses qtkeychain to store password in the dekstop available  keyring services.
Fixes: #6 

The library [Python Keyring Lib] (https://bitbucket.org/kang/python-keyring-lib) supports the following backends:

 * OSXKeychain: supports the Keychain service in Mac OS X.
 * KDEKWallet: supports the KDE's Kwallet service.
 * GnomeKeyring: for Gnome 2 environment.
 * SecretServiceKeyring: for newer GNOME and KDE environments.

Besides these native password storing services provided by operating systems. Python keyring lib also provides following build-in keyrings.

 * Win32CryptoKeyring: for Windows 2k+.
 * CryptedFileKeyring: a command line interface keyring base on PyCrypto.
 * UncryptedFileKeyring: a keyring which leaves passwords directly in file.

I could only test it unser KDE with kwallet.
Unfortuatelly the kwallet backend uses another folder in kwallet so i can't reuse the password stored from the owncloud-client. But there is actually an open issue: https://bitbucket.org/kang/python-keyring-lib/pull-request/21