Use haxe logo as icon

I think using the haxe logo makes the most sense. I tried some variation on it (a book in the middle, as a symbol for "lib", similar to the [flixel-docs logo](https://camo.githubusercontent.com/1c298c6d0f339912602b5362a5d6c22963c96993/68747470733a2f2f7261772e6769746875622e636f6d2f48617865466c6978656c2f68617865666c6978656c2e636f6d2f6d61737465722f7372632f66696c65732f696d616765732f666c6978656c2d6c6f676f732f666c6978656c2d646f63732e706e67)), but that didn't look good at small sizes.