Add auto bounding box

If there is a property called boundingBox on any layer,
the maxX and maxY of the boundingBox property will be
the width and height of that layer. The minX and minY are 0
Merge pull request #118 from joseadrian/master

Add auto bounding box in Tiled by setting the "boundingBox" property on a layer, with value "true" or 1.