Added Assets / Updated CSS (SASS)

Added image assets are used in base64 within the CSS file, binary files
are only added for archive purposes.
Implemented Variable Access to Setter / Getter

* Implemented class variable access with setter / getter methods
throughout the whole project
* Moved fronted / backend functions in their own respective classes
* Updated "include_template" method for future use with user templates
Added / Updated Stylesheets

* Added new backend stylesheet (admin.sass / .css)
* Updated main.css
* Updated README.md