Expression support, including ChangeVar effect
 - Can parse, evaluate and serialize strongly-typed expressions
 - Implements most common operations (easily extensible)
 - Caches constant results to avoid re-evaluation
 - Throws nice exceptions to complain about runtime evaluation problems

Expressions can be built by parsing a lisp-like representation
(see Parser.parse()) or composing Literals, VariableRefs, and Operations

Expressions are evaluated by calling evaluate() on a given VarsContext
Rename expresion.ops to expresion.operators
Change operators visibility to package visilibity.
Moving OperatorFactory to expressions.operators to act as entry point of this package.
Adding prefix "Abstract" to abstract base operator classes.
RRenaming Expression#updateEvaluation to Expression#update
Renaming ExpressionException to ExpressionEvaluationException
Making OperatorFactory#createOperation not static due its use as a method of class instance.
Removing debugging message
Make safer comparisons
Merge pull request #1 from imartinezortiz/expressions

Some minor modifications in Expressions

Thanks for fixing the System.err.println - somehow forgot it during testing. The renames and refactorings are also nice, specially moving the OperatorFactory into the operations subpackage.
The "Make safer comparisons" patch is redundant (see comments), but should not impact performance much.
Merge pull request #53 from manuel-freire/conditions

Expression support