Rename legacy theme support string to 'thematic_xhtml'

The phrase legacy is used in so many places, including
references to pre-1.0 code. Using 'thematic_xhtml' is
more clear to what it does.
Refactor `thematic_is_legacy_xhtml()` to check for theme_supports

Let child themes decide which markup they want using add_theme_support
calls. Fallback is to use xhtml markup on upgraded sites and html5
markup on new installs.

See #111
Hide the checkbox for xhtml markup by default

Keep the possibility to show it via filtering.

See #111
Unit tests: add unit test file for theme options

See #111
Remove call to add_theme_support('thematic_xhtml') from theme setup function

Redundant. We don't need to set it, that's up to child themes
Unit tests: move theme options methods to parent Thematic_UnitTestCase class
Unit test: Rename file to test-helpers.php

Because that's where the functions we are
testing comes from.
Merge pull request #113 from ThematicTheme/feature/html5_theme_support

Feature/html5 theme support

Close #111