Add assert helpers

While correcting an issue over at generator-ember, the need came up for a test to make sure that compass was not installed when the user declines to install compass at the prompt.  There was no convenient helper method available to assert the *absence* of a regex match (needed here for _package.json and Gruntfile.js), so I added it under the name `assertNoMatch`. It also seemed as if the regex form of `assertFile` should be aliased to `assertMatch` for symmetry.