XMLGameEngine

An engine for describing games built around tinyxml2, exprtk, and SFML.

XMLGameEngine dependencies:
* tinyxml2 https://github.com/leethomason/tinyxml2
* exprtk https://github.com/ArashPartow/exprtk
* SFML https://github.com/SFML/SFML

XMLGameEngine stores a complete description of a game in an XML.
The XML file is loaded and parsed by tinyxml2.
Any variables, expressions, or function calls in the XML data is evaluated by exprtk.
The resulting values and objects are used to render the game with SFML.

XMLGameEngine is currently an alpha, and not complete.

Things that work:
* Loading and parsing XML
* Expression and function evaluation
* Creation of variables, objects, and states
* Key handling for objects and states
* Some drawing functions: shape.circle(), shape.rectangle()
* Basic text rendering: text()
* Some collision: screen boundaries, circle-rectangle

Things still missing:
* Collisions of all types
* Scoring system
* Win Condition