First SDK release

This is the first initial release of the Research Arcade SDK for Unity. The release contains the following features:

• Class for detecting arcade bindings in three states: held down, pressed and released.
• Syntactic sugar for checking bindings.
• Navigation methods for exiting game on WebGL, back to main menu of the Research Arcade.
• An example testing class you can set up with RawImages. See the test scene in the main repo for a better example of this.