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.
Added a builder for creating GiniSDK instances.
SDK Builder: Expose the injector.
Merge pull request #22 from gini/gini_user

Anonymous users.