Issue #53 - Add User class

* Create application-wide User
iSsue #53 - Add new navigation stack Activities

* Add new Activities
* Update AndroidManifest
Issue #53 - Adapt Fragments to new Activities
Issue #53 - Update Action Bar options menu

* Add icons
* Add new menu files
Issue #53 - Update application theme

* Create API > 14 values folder
* TODO: Adjust color-scheme