Different versions tests were update

* test_new_client test was updated
(with new elliptics.Session.add_remote logic)
* raises_elliptics_error matcher was added
(to be able to check exception type and exception code with the same
assert_that construct)
* relative import was corrected
(to make this import more explicit)
* usage of matches method was replaced with _matches method
('virtual' method _matches has to be used as it's implemented in
BaseMatcher class with NotImplementedError exception)
* defining for HasItems' attribute was moved to __init__
* unused imports were removed

reviewers: @uvizhe