Merge pull request #58 from Nagadum/bugfix

Bug correction : enumerate and xrange
Optimizations using iterators :
* Analyse of potential iterators, even in user-created functions.
* Transformation of List Comprehensions intos map(...) when possible
* Transformations of intrinsics into iterators (ex : map -> imap)
* Transformation of remaining List Comprehensions into Generator Expressions
* Docstring with each transformation
* Test cases