Added method setDefinitions(Collection<PropertyDef>) to IWorldTree
    - Changed WorldTree.setDefinitions() modifier to public
Working on resolving dependencies among property definitions
Driver Tweaks
Merging in new driver code.
Merge branch 'gurupras@buffalo.edu' of https://github.com/okennedy/worldtree into gurupras@buffalo.edu

Conflicts:
	src/Driver.java
Merge branch 'gurupras@buffalo.edu' into working
Fixed bugs in Expr class
    - Removed baseExpr field
    - Created Expr object expected to be baseExpr
Updated circular dependency exception message in BasicSolver
Fixed issue with BasicSolver implicitly choosing the underlying
property while aggregating

Example:
    DEFINE ROOM R.treasure as AGGREGATE SUM(REG.monster) in REGION REG;
While computing the value of R.treasure, the solver was implicitly
selecting REG.treasure instead of using the property provided
Merge branch 'gurupras@buffalo.edu' of github.com:okennedy/worldtree into gurupras@buffalo.edu