Add a couple of starter unit tests.

Hey, thanks for writing cssmin!

In this pull request, I added a little test suite and a couple of starter unit tests:
* a test for remove_comments()
* a test for remove_unnecessary_whitespace().

Is this helpful, and if so, do you want more tests for the rest of cssmin? I'm happy to write more if you want.

Or if this needs work/edits, let me know what to fix and I'll try my best :)