ExpressionHelper: Introduce Variables_Identifier

Variables_Identifier provides a way to identify redundant pattern
(ie. variables) in an expression, replace them and obtain the
variables list in a dependencies compliant order.
Test: Add a regression test for Variables_Identifier
VariablesIdentifier: Use iterators (credits Pierre Lalet)
Merge pull request #18 from commial/feature-vars-ident

Feature vars ident