tiled loading for hanging bags

[davidjtferguson]

Now got basic hanging bags, need to be able to load them from tiled with the following flags:

xSpawn: x location for the 'anchor' object the bag is hanging from is
ySpawn: x location for the 'anchor' object the bag is hanging from is
ropeLength: distance between the spawn point and the actual bag
bagWidth: width of the bag
bagHeight: height of the bag
pivitingJoint: if true, add another joint between the bag and the end of the 'rope' so the bag rotates around the point where it meets the rope. Otherwise keep it as is where the bag rotates around the anchor

[davidjtferguson]

Completed in pull request #24