Version move to 2.0.0a1
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 #44 into release-1.8 -- add 'not_' alias for is_not