Add not_ alias of is_not for better readability of negations

Example:

>> assert_that(alist, is_not(has_item(item)))

can be

>>assert_that(alist, not_(has_item(item)))
Add not_ alias of is_not for better readability of negations

Add a `not_` alias inside the core.isnot module and update docs
Merge pull request #44 from naufraghi/patch-2

Add not_ alias of is_not for better readability of negations (2nd style option)