Add a layer between Corona and the game

[aurelien-defossez]

Add a layer to create a more flexible API.

e.g. instead of
self.xpCounter = display.newText(self.player.xp, 0, 0, native.systemFontBold, 32)
self.xpCounter:setReferencePoint(display.MiddleCenterReferencePoint)
self.xpCounter:rotate(self.player.direction)
self.xpCounter.x = self.width / 2
self.xpCounter.y = self.height / 2
self.xpCounter:setTextColor(0, 0, 0)
self.group:insert(self.xpCounter)

Do something like
createText{
text = self.player.xp,
size = 32,
reference = display.MiddleCenterReferencePoint,
x = self.width / 2,
y = self.height / 2,
group = self.group
}