add xstream converter, error during loading
added xstream-1.4.7.jar to libs folder
Modified initialization of ConcurrentFormulaHashMap.java
Fixed load projects error

TODO: fix loading past projects
Introduced CatroidXStream.java
FIXED loading old projects
TODO check value of formula element when converting
FIXED check value of formula element when converting
Removed clone method from SetXBrick and modified to support clone
Modified CatroidXStream
Removed clone method from PlaceAtBrick and modified to support clone
Modified CatroidXStream