Rename AttachTexture in SetTexture
Use slices in Group instead of maps
Add Move for relative translations of shapes
Add SetCenter for setting a new center of the shape
Add Clone for cloning the shape
Improve documentation
Fix tests build (two tests are failing, though)
Fix TestShape
Fix TestGroup
Add RotateAround method in order to make rotation around a given point
Rotations sum angles. Bugfix in tests, pre-rotated box in TestRotatedGroup is rotated by 10.0, not 20.0.