Improve documentation in GINISession class
Implemented a keychain manager.
Replaced the KeychainItemWrapper with our KeychainManager implementation.
Merge pull request #21 from gini/session_doc_improvements

Improve documentation in GINISession class
Merge pull request #20 from gini/Keychain

Keychain
Add model for GINIUser
New factory for the `GINIUser` model for GINI User Center API responses.
Implemented the GINIUserCenterManager.
GINICredentialsStore: Added the possibility to store user login credentials.
Implemented a session manager for anonymous accounts.