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.
Make sure 2.0 default options get set when upgrading from pre 1.0

Add 2.0 theme options to array returned by thematic_convert_legacy_opt()
Unit tests: add tests for theme options

See #62
Merge pull request #113 from ThematicTheme/feature/html5_theme_support

Feature/html5 theme support

Close #111