#1 Improve the Expression class

[Stefano Liboni]

The Expression class in Multiform Framework is used to evaluate the expressions that developer may put into the XML-based language for Process definition.
It has 1 single method called evaluate that has for input the expression found in the XML-based language and returns the value of that expression.
The syntax of the expression and the transformation needed are specified in the file Expression.js in the head comment.
The class is already used by the other classes and the Process compiler doesn't touch the expressions, so changing Expression will require the developer to use the new easyer syntax.