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
Added workaround for a bug introduced by a recent change to Expr class

The removal of baseExpr field from Expr class led to storing all of
baseExpr fields within 'this'. This led to issues especially with
the retrieval of Expr.property (see FIXME in Expr.value())

This commit forces 'this' to take baseExpr.property as its property
in the ARITH constructor case
Working on basic expression handling
Added comments to container classes
Merge branch 'gurupras@buffalo.edu' of github.com:gurupras/worldtree into gurupras@buffalo.edu
Merge branch 'gurupras@buffalo.edu' into working
Added missing interface condition() to IExpr

Updated Expr class' constructors to use baseExpr fields for all
of this' fields wherever applicable
Resolved a FIXME in Expr.value() method by removing the controversial
piece of code
BasicSolver now has code to handle expressions

This is in no way a complete solution and is almost guaranteed to
have bugs. However, it is a decent platform to build up from.
Added sample files for simple dependency in definitions
Removed erroneous break statement in BasicSolver.evaluate()
Added sample files for complex aggregate expressions
Fixed minor bugs in BasicSolver

Commented all private methods of BasicSolver
Updated some sample definition filenames
Merge branch 'working' of github.com:okennedy/worldtree