[strutils] implements natural_enumerate

[mgaitan]

Return a string with a natural enumeration of the given items.

It's based on my contribution to IPython
https://github.com/ipython/ipython/blob/3ced24d6b07c3d66bbae5bef24fa59af25606512/IPython/utils/text.py#L729

which is an extended version of Django's get_text_list()`
https://github.com/django/django/blob/0ee2b8c326d47387bacb713a3ab369fa9a7a22ee/django/utils/text.py#L270

    >>> natural_enumerate(['a', 'b', 'c', 'd'])
    'a, b, c and d'
    >>> natural_enumerate(['a', 'b', 'c'], ' or ')
    'a, b or c'
    >>> natural_enumerate(['a', 'b', 'c'], ', ')
    'a, b, c'
    >>> natural_enumerate(['a', 'b'], ' or ')
    'a or b'
    >>> natural_enumerate(['a'])
    'a'
    >>> natural_enumerate([])
    ''
    >>> natural_enumerate(['a', 'b'], wrap_item_with="`")
    '`a` and `b`'
    >>> natural_enumerate(['a', 'b', 'c', 'd'], " = ", sep=" + ")
    'a + b + c = d'

I'm open to find a better name

[mgaitan]

I refuse to make this code 2.7 compatible. #339 should be implemented.