Use minitest rather than test/unit

Ruby Haml uses minitest as its test runner, and incorporates haml-spec
into its tests. Combining test/unit and minitest in the same process
is problematic and causes several problems.
Merge pull request #6 from mattwildig/minitest

Use minitest rather than test/unit
Don't assume XHTML by default
Only wrap filter content in CDATA with XHTML
Merge branch 'master' of github.com:haml/haml-spec