Added assert_empty, assert_not_empty, and tests
Changed assert_not_empty to use else on a for loop
Better error message for assert_empty

Also, cleaned up imports
Better assert_empty message; testing assert_empty's message; assert_raises_particular_exception and tests
Merge branch 'master' of https://github.com/Yelp/Testify into assert_empty_and_assert_not_empty
Changes re:Tyler's review, incl. assertion rename

From assert_raises_particular_exception to assert_raises_such_that
Merge pull request #117 from Jebdm/assert_empty_and_assert_not_empty

Added assert_empty, assert_not_empty, assert_raises_particular_exception, and tests