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)
ExpressionHelper: Introduce ExprRandom, an expression generation feature

ExprRandom is a customisable (by extending it) class for generating
random expression.
Example: Add an example for ExprRandom
Merge pull request #18 from commial/feature-vars-ident

Feature vars ident
Merge branch 'feature-exprrandom' of https://github.com/commial/miasm into commial-feature-exprrandom

Conflicts:
	miasm2/expression/expression_helper.py