Added method setDefinitions(Collection<PropertyDef>) to IWorldTree
    - Changed WorldTree.setDefinitions() modifier to public
Working on resolving dependencies among property definitions
Added validateDefinitions method to ConstraintSolver

This method is to be used before pushing down constraints
It ensures that all the constraints that have been defined, have
definitions at every lower level
Merge branch 'gurupras@buffalo.edu' of github.com:gurupras/worldtree into gurupras@buffalo.edu
Merge branch 'gurupras@buffalo.edu' into working
Merge branch 'working' of https://github.com/okennedy/worldtree