Add not_ alias of is_not for better readability of negations
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)))
nitishr/PyHamcrest,msabramo/PyHamcrest,msabramo/PyHamcrest,nitishr/PyHamcrest