Python3 version compatible

List of changes (non exhaustive)
- print --> print()
- exec --> exec()
- Cpickle --> pickle for Python3 and Cpickle for Python2
- six module importation in order to reproduce xrange() without breaking compatibility with Python2
- relative module importation (consequence: skrf must be imported elsewhere from its own directory)
- some range() --> list(range()) since Python range() returns now iterators
- iteritem() calls removed and replaced by 'in'
- for Python2: import from future: division, relative_import, print_function