* Add CredentialStore API
* Add CredentialStore implementation for GNOME keyring (makes GNOME keyring dependency optional)
* Further CredentialStore implementations (e.g. KDE) may be added to CredentialStore.py
* Mailnag tries to detect the supported CredentialStore backend automatically
* If no CredentialStore backend is supported on the system, Mailnag stores passwords in the config file (plaintext)
* A specific CredentialStore backend can be forced in the config file (currently one of 'auto', 'gnome', 'none')
* Closes issue #82
