Revamp py.test's handling of test files.

Each .scss file is now its own test -- you can run them directly, with
`py.test scss/tests/files/foo/bar.scss`, or use wildcards, or specify a
partial directory, or whatever.

Accordingly, `test_files.py` is gone, as is the `--test-file-filter`
option that was so cumbersome I never actually used it.

Regressions:
- xfail is no longer supported.
- Ruby tests are now not collected at all, rather than marked as
  skipped.
Cut down on the punctuation noise in Sass reprs.
Make a best-effort to patch up some gradient stuff.  Fixes #277, #292.